Add branch and tag info

CONTRIBUTING.md

@@ -4,6 +4,25 @@ Please do contribute!
 
 [See the wiki](https://github.com/calmh/syncthing/wiki/Building)
 
+## Branches
+
+ - `master` is the main branch containing good code that will end up in
+   the next release. You should base your work on it. It won't ever be
+   rebased or force-pushed to.
+
+ - `vx.y` branches exist to make patch releases on otherwise obsolete
+   minor releases. Should only contain fixes cherry picked from master.
+   Don't base any work on them.
+
+ - Other branches are probably topic branches and may be subject to
+   rebasing. Don't base any work on them unless you specifically know
+   otherwise.
+
+## Tags
+
+All releases are tagged semver style as `vx.y.z`. Release tags are
+signed by GPG key BCE524C7.
+
 ## Tests
 
 Yes please!