docs: Document procedure for updating pull requests

See discussion in #56637.

Reviewed By: ldionne, jhenderson

Differential Revision: https://reviews.llvm.org/D147284

Author: tstellar

llvm/docs/GitHub.rst

@@ -19,6 +19,30 @@ is reserved for official project branches only.  We may relax this rule in
 the future if needed to support "stacked" pull request, but in that case only
 branches being used for "stacked" pull requests will be allowed.
 
+Pull Requests
+=============
+The LLVM Project does not currently accept pull requests for the llvm/llvm-project
+repository.  However, there is a
+`plan <https://discourse.llvm.org/t/code-review-process-update/63964>`_ to move
+to pull requests in the future.  This section documents the pull request
+policies LLVM will be adopting once the project starts using them.
+
+Creating Pull Requests
+^^^^^^^^^^^^^^^^^^^^^^
+For pull requests, please push a branch to your fork of the llvm-project and
+`create a pull request from the fork <https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request-from-a-fork>`_.
+
+Updating Pull Requests
+^^^^^^^^^^^^^^^^^^^^^^
+When updating a pull request, you should push additional "fix up" commits to
+your branch instead of force pushing.  This makes it easier for GitHub to
+track the context of previous review comments.
+
+If you do this, you must squash and merge before committing and
+you must use the pull request title and description as the commit message.
+The default commit message for a squashed pull request is the pull request
+description, so this will allow reviewers to review the commit message before
+approving the commit.
 
 Releases
 ========