BRANCHES.md: Add explanation of branches and using forks

BRANCHES.md

@@ -0,0 +1,59 @@
+Branches used in this repo
+==========================
+
+The document explains the branching structure that we are using in the VFSForGit repository as well as the forking strategy that we have adopted for contributing.
+
+Repo Branches
+-------------
+
+1. `vfs-#`
+
+    These branches are used to track the specific version that match Git for Windows with the VFSForGit specific patches on top.  When a new version of Git for Windows is released, the VFSForGit patches will be rebased on that windows version and a new gvfs-# branch created to create pull requests against.
+
+    #### Examples
+
+    ```
+    vfs-2.27.0
+    vfs-2.30.0
+    ```
+
+    The versions of git for VFSForGit are based on the Git for Windows versions.  v2.20.0.vfs.1 will correspond with the v2.20.0.windows.1 with the VFSForGit specific patches applied to the windows version.
+
+2. `vfs-#-exp`
+
+   These branches are for releasing experimental features to early adopters. They
+   should contain everything within the corresponding `vfs-#` branch; if the base
+   branch updates, then merge into the `vfs-#-exp` branch as well.
+
+Tags
+----
+
+We are using annotated tags to build the version number for git.  The build will look back through the commit history to find the first tag matching `v[0-9]*vfs*` and build the git version number using that tag.
+
+Full releases are of the form `v2.XX.Y.vfs.Z.W` where `v2.XX.Y` comes from the
+upstream version and `Z.W` are custom updates within our fork. Specifically,
+the `.Z` value represents the "compatibility level" with VFS for Git. Only
+increase this version when making a breaking change with a released version
+of VFS for Git. The `.W` version is used for minor updates between major
+versions.
+
+Experimental releases are of the form `v2.XX.Y.vfs.Z.W.exp`. The `.exp`
+suffix indicates that experimental features are available. The rest of the
+version string comes from the full release tag. These versions will only
+be made available as pre-releases on the releases page, never a full release.
+
+Forking
+-------
+
+A personal fork of this repository and a branch in that repository should be used for development.
+
+These branches should be based on the latest vfs-# branch.  If there are work in progress pull requests that you have based on a previous version branch when a new version branch is created, you will need to move your patches to the new branch to get them in that latest version.
+
+#### Example
+
+```
+git clone <personal fork repo URL>
+git remote add ms https://github.com/Microsoft/git.git
+git checkout -b my-changes ms/vfs-2.20.0 --no-track
+git push -fu origin HEAD
+```