message: docs: update release changelog generation process.
  This uses `yarn -s` to strip unnecessary wrapper text and uses `--type` instead of `--mode`, which is the required flag for `ng-dev`. The instructions now call out that two different release notes need to be generated in two different places.
diff: @@ -36,12 +36,30 @@ git push upstream && git push upstream --tags
  # Release Changelog
  +Release changelog needs to be included in two places, the GitHub release and in
  +the
  +[`CHANGELOG.md`](https://github.com/angular/universal/blob/master/CHANGELOG.md)
  +file. These use different formats, so start with GitHub by running:
  +
  +```sh
  +yarn -s ng-dev release notes --type=github-release --from=<The git tag or ref to start the changelog entry from>
  +```
  +
  +- Go to the [`Releases` tab](https://github.com/angular/universal/releases) in
  + the repository.
  +- Click on the newly created tag.
  +- Copy paste the output of the above command into the contents of the release.
  +- Choose `This is a prerelase` checkbox for pre-release versions.
  +
  +After creating a GitHub release, update the
  +[`CHANGELOG.md`](https://github.com/angular/universal/blob/master/CHANGELOG.md)
  +file by prepending it with changelog-formatted release notes (note
  +`--type=changelog` which is _different_ from the GitHub release):
  +
  ```sh
  -yarn ng-dev release notes --from=<The git tag or ref to start the changelog entry from>
  -yarn ng-dev release notes --mode=github-release --from=<The git tag or ref to start the changelog entry from>
  +cat <(yarn -s ng-dev release notes --type=changelog --from=<The git tag or ref to start the changelog entry from>) \
  + CHANGELOG.md > CHANGELOG.md
  ```
  -- Go to the `Releases` tab in the repository
  -- Click on the newly created tag (9.0.0-next.13 in this case)
  -- Paste the contents of clipboard to the contents
  -- Choose `This is a prerelase` checkbox for pre-release versions
  +Then make a PR with the update targeting `master`, get a team member to review
  +it, and merge when ready.
  