Define a release cadence

[muvaf]

What problem are you facing?

Crossplane has its own release cadence but that doesn't bind the providers. We need a similar mechanism for provider-gcp.

How could Crossplane help solve your problem?

So far, we released providers when we released a new Crossplane version. We could continue doing that or have a separate release cycle but it's important to document it in the README.md.

I'd suggest to release provider-gcp every other month similar to Crossplane.