Update .github/CONTRIBUTING.md

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>
Signed-off-by: Jonathan D.A. Jewell <6759885+hyperpolymath@users.noreply.github.com>

Author: hyperpolymath

.github/CONTRIBUTING.md

@@ -298,5 +298,5 @@ unsigned pushes. Estate policy:
   so that GitHub signs each commit.
 - Merge PRs with **squash**. The ruleset checks every commit on the PR branch,
   not just the result, so one unsigned commit blocks the merge. Re-create such a
-  branch with signed commits (`git cherry-pick -S`) and open a new PR.
+  branch with signed commits (`git cherry-pick -S`) and update the existing PR branch if permitted. Open a new PR only if repository rules prevent that update.
   Rebase-merge replays commits unsigned and is disabled.