Add git rolling instructions directly to depot_tools.

bootstrap/win/README.md

+# Windows binary tool bootstrap
+
+This directory has the 'magic' for the `depot_tools` windows binary update
+mechanisms.
+
+## Software bootstrapped
+  * Python (https://www.python.org/)
+  * Git for Windows (https://git-for-windows.github.io/)
+  * Subversion (https://subversion.apache.org/)
+
+## Mechanism
+
+Any time a user runs `gclient` on windows, it will invoke the `depot_tools`
+autoupdate script [depot_tools.bat](../../update_depot_tools.bat). This, in
+turn, will run [win_tools.bat](./win_tools.bat), which does the bulk of the
+work.
+
+`win_tools.bat` will successively look to see if the local version of the binary
+package is present, and if so, if it's the expected version. If either of those
+cases is not true, it will download and unpack the respective binary.
+
+Downloading is done with [get_file.js](./get_file.js), which is a windows script
+host javascript utility to vaguely impersonate `wget`.
+
+Through a comedy of history, each binary is stored and retrieved differently.
+
+### Git
+
+Git installs are mirrored versions of the offical Git-for-Windows Portable
+releases.
+  * Original: `https://github.com/git-for-windows/git/releases`
+  * Mirror: `gs://chrome-infra/PortableGit*.7z.exe`
+
+#### Updating git version
+  1. Download the new `PortableGit-X.Y.Z-{32,64}.7z.exe` from the
+     git-for-windows release page.
+  1. From either console.developers.google.com or the CLI, do:
+    1. Upload those to the gs://chrome-infra Google Storage bucket.
+    1. Set the `allUsers Reader` permission (click the "Public link" checkbox
+       next to the binaries).
+  1. Edit the `set GIT_VERSION=X.Y.Z` line in `win_tools.bat` to be the new
+     version.
+    1. At the time of writing, the first version is the default version, and
+       the second is the 'bleeding edge' version. You can use the bleeding edge
+       version to get early feedback/stage a rollout/etc.
+  1. Commit the CL.
+
+Note that in order for the update to take effect, `gclient` currently needs to
+run twice. The first time it will update the `depot_tools` repo, and the second
+time it will see the new git version and update to it. This is a bug that should
+be fixed, in case you're reading this and this paragraph infuriates you more
+than the rest of this README.
+
+### Python
+
+Python installs are sourced from https://src.chromium.org/viewvc/chrome/trunk/tools/ .
+
+The process to create them is sort-of-documented in the README of the python
+zip file.
+
+### Subversion
+
+Subversion installs are sourced from https://src.chromium.org/viewvc/chrome/trunk/tools/ .
+
+There will likely never be an update to SVN in `depot_tools` from the current
+version.