Improve release process steps in CONTRIBUTING.md

Update the `poetry publish` step to mention that a PyPi API token is
needed and provide instructions on how to set it up.
Add a step about updating https://changelog.ably.com/ via headway app.

CONTRIBUTING.md

@@ -39,9 +39,10 @@ The release process must include the following steps:
 6. Push the release branch to GitHub
 7. Create a release PR (ensure you include an SDK Team Engineering Lead and the SDK Team Product Manager as reviewers) and gain approvals for it, then merge that to `main`
 8. Build the synchronous REST client by running `poetry run unasync`
-9. From the `main` branch, run `poetry build && poetry publish` to build and upload this new package to PyPi
+9. From the `main` branch, run `poetry build && poetry publish` (will require you to have a PyPi API token, see [guide](https://www.digitalocean.com/community/tutorials/how-to-publish-python-packages-to-pypi-using-poetry-on-ubuntu-22-04)) to build and upload this new package to PyPi
 10. Create a tag named like `v2.0.1` and push it to GitHub - e.g. `git tag v2.0.1 && git push origin v2.0.1`
 11. Create the release on GitHub including populating the release notes
+12. Update the [Ably Changelog](https://changelog.ably.com/) (via [headwayapp](https://headwayapp.co/)) with these changes
 
 We tend to use [github_changelog_generator](https://github.com/skywinder/Github-Changelog-Generator) to collate the information required for a change log update.
 Your mileage may vary, but it seems the most reliable method to invoke the generator is something like: