docs/CONTRIBUTING: Update Polkadot pull request process documentation (#7393)

* docs/CONTRIBUTING: Update Polkadot pull request process

* Update docs/CONTRIBUTING.adoc

Co-authored-by: David <dvdplm@gmail.com>

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>
Co-authored-by: David <dvdplm@gmail.com>

substrate/docs/CONTRIBUTING.adoc

@@ -71,8 +71,15 @@ To create a Polkadot companion PR:
 . Make the changes required and build polkadot locally.
 . Submit all this as a PR against the Polkadot Repo. Link to your Polkadot PR in the _description_ of your Substrate PR as "polkadot companion: [URL]"
 . Now you should see that the `check_polkadot` CI job will build your Substrate PR agains the mentioned Polkadot branch in your PR description.
-. Wait for reviews on both
-. Once both PRs have been green lit, they can both be merged 🍻.
+. Wait for reviews on both the Substrate and the Polkadot pull request.
+. Once the Substrate pull request runs green, a member of the `parity` github group can comment on the Substrate pull request with `bot merge` which will:
+    - Merge the Substrate pull request.
+    - In case the pull request origins from https://github.com/paritytech/polkadot directly and not from a fork:
+        - The bot will push a commit to the Polkadot pull request updating its Substrate reference.
+        - The bot will merge the Polkadot pull request once all its CI checks are green.
+    - In case the pull request origins from a fork (relevant for pull requests by external contributors):
+        - You need to push a commit to the Polkadot pull request updating the Substrate reference.
+        - You need to merge by commenting `bot merge` on the Polkadot pull request once all CI checks on the pull request are green.
 
 If your PR is reviewed well, but a Polkadot PR is missing, signal it with https://github.com/paritytech/substrate/labels/A7-needspolkadotpr[`A7-needspolkadotpr`] to prevent it from getting automatically merged.