Merge pull request #211: Documentation: add docs around testing installers

The way we test pull requests in the microsoft/git fork is a bit
strange. We rely on the integration between the Git code and a C#
layer in another repository. The functional tests in the C# layer
are interesting and sometimes the only way to fully test the
complex interactions between the components.

Document the process for generating and using an installer.

Documentation/microsoft-git.md

@@ -0,0 +1,48 @@
+Details about the Microsoft/Git Fork
+====================================
+
+The [`microsoft/git` fork of Git](https://github.com/microsoft/git)
+is used to take early performance patches or custom patches for use with
+[VFS for Git](https://github.com/microsoft/vfsforgit).
+
+Integration Testing with C# Layer
+---------------------------------
+
+If you have a pull request in `microsoft/git` and it passes all the normal
+Git-specific builds and tests, then you can test integration in VFS for Git
+using a pull request there.
+
+1. First, build a custom Git installer for your PR merge commit. The GVFS test
+   suite will install this on the test machine and use it to support the GVFS
+   functional tests. The installer packages will be posted to a private nuget
+   feed. The installer build step it in a different repo that is unknown to
+   `microsoft/git`. Run the [`git - build installers`](https://dev.azure.com/gvfs/ci/_build?definitionId=11)
+   using "Branch/tag" as `refs/pull/<pr-number>/merge`. Leave
+   "package.preview.extension" as `-pr`. Note the build number. It will have
+   the form `2.YYMMDD.#`.
+
+2. Create a topic branch in VFS for Git to consume your custom built Git installer.
+   Modify the [`GVFS/GVFS.Build/GVFS.props`](https://github.com/microsoft/VFSForGit/blob/master/GVFS/GVFS.Build/GVFS.props)
+   file to change the `<GitPackageVersion>` to be the build number of the installer
+   build, followed by the extension (`-pr`), such as `2.YYMMDD.#-pr`. Include
+   "[PR Build]" in your commit message, to make it clear this is a build based
+   on unmerged commits.
+
+3. Create a pull request in VFS for Git and the functional tests will run. (Make sure
+   the installer build has finished before doing this.) Include "[PR Build]" in your
+   pull request title.
+
+4. Repeat the previous three instructions as you update your `microsoft/git`
+   pull request. Those steps are all manual, so be sure that you have run the
+   VFS for Git functional tests based on an up-to-date build of your Git pull
+   request.
+
+5. After merging your pull request in `microsoft/git`, you can generate an
+   installer build based on the target branch.
+
+   i. For a `vfs-2.*` branch, set the "package.preview.extension" to be empty.
+   ii. For the `features/sparse-checkout-2.*` branch, set the "package.preview.extension" to "-sc".
+
+6. Amend your commit in VFS for Git to point to the build number of your "release" build.
+   Remove the "[PR Build]" from your commit message and pull request title. Merge that
+   pull request when ready.