Updater: Add code documentation Markdown file

Add docs/autoupdate_overview.md which gives an overview of the update
process, and comments pointing to it in autoupdate related files.

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+# AutoUpdate Overview
+Dolphin's autoupdate procedure is spread through a number of files; this overview describes the
+update flow.
+
+## General notes:
+* The updater is only supported on Windows and MacOS.
+* There are four update frequency tracks: Dev (updated every commit), Beta (a few times a year),
+  Stable (very rarely), and Disabled.
+* Applications can't overwrite themselves so a separate application is responsible for actually
+  updating the Dolphin executable and other files.
+* The updater application needs to be able to also update itself, so it creates a copy which
+  updates both Dolphin and the original updater. Every time Dolphin launches it deletes the
+  updater copy (if it exists).
+
+## Class and file responsibilities:
+* AutoUpdateChecker (UICommon/AutoUpdate.h): Checks if an update is available.
+    * Verifies the updater is supported on the user's platform and hasn't been disabled.
+    * Retrieves new version information from the update server.
+    * Copies the updater application and launches the copy.
+* Updater (DolphinQt/Updater.h): Serves as the interface between AutoUpdateChecker and Qt.
+    * Spawns a background thread when Dolphin launches that calls AutoUpdateChecker.
+    * Creates the update prompt window when an update is available.
+    * If the user wants to update now, closes Dolphin.
+* MacUpdater/main.m and MacUpdater/AppDelegate.mm: Entry point to MacOS updater.
+    * Converts command line arguments to vector\<string\> and passes them to
+      UpdaterCommon::RunUpdate().
+* WinUpdater/main.cpp: Entry point to Windows updater.
+    * Converts command line arguments to vector\<string\>.
+    * Checks if updater has write access to the Dolphin executable directory. If not, attempts
+      to relaunch itself with admin privileges (creating a User Account Control prompt).
+    * Passes argument vector to UpdaterCommon::RunUpdate().
+* UpdaterCommon/UpdaterCommon.cpp: Performs the actual update process.
+    * Manages updater UI.
+    * Fetches file manifests from update server for current and new versions.
+    * Calculates file diff between versions.
+    * Downloads and adds/replaces changed files and deletes removed files.
+    * Verifies file hashes.
+    * If the user updated immediately (rather than waiting for Dolphin to close before starting
+       the update), starts Dolphin again when the update is complete.
+
+## Update flow:
+* An update check is started in one of two ways:
+    * When Dolphin is launched (unless in NoGUI or batch mode):
+         * In main.cpp an instance of Updater is created and invokes start(), which is inherited
+           from QThread and creates a new thread which performs the check off the main thread.
+         * QThread::start() calls run() which is overridden in Updater and calls
+           AutoUpdateChecker::CheckForUpdate().
+    * When the user selects Help -> "Check for Updates..." in the main menu:
+         * The menu option runs a callback to MenuBar::InstallUpdateManually().
+         * The Config value for the update track is backed up to a temporary variable, then
+           overwritten with "dev" to force a check for the latest version. After the check
+           completes the original Config value is restored.
+         * Updater::CheckForUpdate() is called, which calls AutoUpdateChecker::CheckForUpdate().
+* AutoUpdateChecker::CheckForUpdate() checks if the selected track has a new version available.
+    * If not the check ends. If the user started it, returns to Updater::CheckForUpdate() which
+      tells the user they're up to date.
+* Information about the update is passed to OnUpdateAvailable(), which is overridden by Updater.
+* OnUpdateAvailable() creates a window displaying the update changelog and asks the user if they
+  want to update now, update after Dolphin closes, not update, or never auto-update.
+* If the user wants to update AutoUpdateChecker::TriggerUpdate() is called.
+* TriggerUpdate() builds the command line arguments for the updater process, creates a copy of
+  the updater executable, then runs the copy in a new process.
+* TriggerUpdate() returns to OnUpdateAvailable().  If the user chose to update now, Dolphin's
+  main window is closed which results in the Dolphin process ending.
+* The updater process begins.
+    * On MacOS (starts main() in MacUpdater/main.m):
+         * Checks that the process received command line arguments. If not it tells the user the
+           updater can't be launched directly and quits.
+         * Calls NSApplicationMain(), which passes control to the AppDelegate defined in
+           MacUpdater/AppDelegate.mm.
+         * The command line arguments are converted to a vector\<string\>
+           and passed to RunUpdater() in UpdaterCommon.h.
+    * On Windows (starts wWinMain() in WinUpdater/Main.cpp):
+         * Checks that the process received command line arguments. If not it tells the user the
+           updater can't be launched directly and quits.
+         * Attempts to open Updater.log in the same directory as Dolphin.exe. If this fails,
+           checks to see if the process has admin privileges.
+             * If not, attempts to relaunch the updater as admin. This will spawn a User Account
+               Control prompt.
+             * If the user declines the UAC prompt, or if the updater already has admin status,
+               the update aborts.
+         * Converts the command line arguments to a vector\<string\> and passes them to RunUpdater()
+           in UpdaterCommon.h.
+* RunUpdater() parses and validates the command line arguments, hides the updater UI, then waits
+  for the Dolphin process to quit.
+* RunUpdater() begins the actual update.
+    * Fetches file manifests from update server for current and new versions.
+    * Decompresses manifests and verifies their signatures.
+    * Downloads and adds/replaces changed files and deletes removed files.
+    * Verifies downloaded files match manifest hashes.
+* If the user updated immediately (rather than waiting for Dolphin to close before starting
+  the update), Dolphin restarts.
+* As part of Dolphin's normal startup process, the Updater copy is deleted.