add phase option, for phase=release

This allows creating the file in the root, but only when the dist is released (and then [Git::Commit] can commit it), so there isn't a dirty file being created on every build.

Currently my pluginbundle is creating the file in the dist, and then [CopyFilesFromRelease] moves the file back to the repo for commit. But then I have an extra README.pod in my dist that doesn't add any value!