Automaticall sync README.md to gh-pages branch

[rainest]

https://charts.konghq.com/ is served from the gh-pages branch. chart-releaser updates this branch with new chart version bundles, but doesn't update anything else.

We display README.md as a landing page there, but it doesn't get updated when the main version does. We'd like to handle this automatically in CI.

Offhand I know how to sync an entire branch, but not a single file, so this is a TODO to figure that out.