message: Update PR template for new changelog stuff
diff: <!-- Please read https://github.com/matrix-org/matrix-js-sdk/blob/develop/CONTRIBUTING.md before submitting your pull request -->
  <!-- Include a Sign-Off as described in https://github.com/matrix-org/matrix-js-sdk/blob/develop/CONTRIBUTING.md#sign-off -->
  +
  +<!-- To specify text for the changelog entry (otherwise the PR title will be used):
  +Notes:
  +
  +Changes in this project generate changelog entries in element-web by default.
  +To suppress this:
  +
  +element-web notes: none
  +
  +...or to specify different notes:
  +element-web notes: <notes>
  +-->
  