Update CONTRIBUTING.md with merge instructions (theatre-js#164)

Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -273,3 +273,9 @@ convention for these branches should be:
 - `(feature|hotfix|docs)/[identifier]` or
 - an autogenerated branch name from a Github issue (On an issue without a PR,
   look under the "Development" sidebar heading for a "create a branch link")
+
+Squash & merge should be preferred most of the time to keep the main branch clean, unless the commit history is relevant, in which case rebase should be used.
+
+Always update your feature branch with a rebase from the main branch to make sure that you don't accidentally break main with an undetected conflict.
+
+Branches belonging to merged PRs should be deleted.