docs: add to RELEASE.md suggestions where to announce a release

Lars Wirzeniusrequested to merge
liw/release-announce into main

Description

Remember to announce releases on blogs.

Checklist

  • You have read, and followed the guidance in, the CONTRIBUTING guide
  • The MR content has run through a pipeline, or at least has pipelines enabled.
  • There are no licence concerns with this contribution (The README clearly states Subplot is MIT-0)

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