[docs] Stress out the branch naming scheme for Graphite.

llvm/docs/GitHub.rst

@@ -23,6 +23,23 @@ intended to be able to support "stacked" pull-request. Do not create any branche
 llvm/llvm-project repository otherwise, please use a fork (see below). User branches that
 aren't associated with a pull-request **will be deleted**.
 
+Using Graphite for stacked Pull Requests
+========================================
+
+[Graphite](https://app.graphite.dev/) is a stacked pull request tool supported
+by the LLVM repo (the other being [reviewable.io](https://reviewable.io)).
+
+Graphite will want to create branches under `llvm/llvm-project` rather than your
+private fork, so the guidance above, about branch naming, is critical, otherwise
+`gt submit` (i.e. publish your PRs for review) will fail.
+
+Use `gt config` then `Branch naming settings` and `Set a prefix for branch names`.
+Include the last `/`.
+
+If you didn't do the above and Graphite created non-prefixed branches, a simple way to
+unblock is to rename (`git -m <old name> <new name>`), and then checkout the branch
+and `gt track`.
+
 Pull Requests
 =============
 The LLVM project is using GitHub Pull Requests for Code Reviews. This document