[Contributing] Document the release process

[liamnichols]

Adding a note here for now because I forgot...

1. Bump the version (example)
2. Download xcstrings-tool.artifactbundle.zip from the workflow run for the version bump commit (example)
   • Make sure that it's not double zipped (unzip -l)
3. Compute it's checksum with shasum -a 256 <path>
4. Create a new release and include the artifactbundle and it's sha (see other releases as an example)
5. Update https://github.com/liamnichols/xcstrings-tool-plugin
   • Update the binaryTarget in Package.swift for the new version
   • Copy the plugin over if it was modified
6. Create a new release in the plugin repository
7. Update https://github.com/liamnichols/xcstrings-tool-demo