Add release notes details to release process docs

doc/release-process.md

@@ -88,6 +88,8 @@ Install deps on Linux:
    - Try to generate checkpoints as close to the release as possible, so you can have a recent block height be protected.
      - For instance, don't update checkpoints and then do a release a month later. You can always update checkpoint data again or multiple times
  - Update copyright years (if applicable) with util/replace.pl
+ - Update doc/relnotes/README.md
+   - To get the stats of file changes: `git diff --stat master...dev`
  - Do a fresh clone and fresh sync with new checkpoints
  - Stop node, wait 20 minutes, and then do a partial sync with new checkpoints
  - Make Gitea release