docs: disallow rebase merges

[njoyard]

Rebase merges produce a git history that does not include PR numbers, which makes github-changelog fail to generate a proper changelog.

Doing a merge with rebase on a repository with release-plan will produce a Prepare release vnull PR with a faulty .release-plan.json and empty changelog:

{
  "solution": {
    "@org/package": {
      "oldVersion": "1.2.3"
    }
  },
  "description": "\n\n\n\n"
}

This PR updates the README and RELEASE.md to suggest disabling rebase merging on their repositories.