Document future of this ruleset

This proposal: https://docs.google.com/document/d/1GW7UVo1s9X0cti9OMgT3ga5ozKYUWLPk9k8c4-34rC4/edit# suggests that rules_cc is going to at least stagnate in development as an internal to Bazel migration occurs.
It'd be useful to document at least some ideas of what the future for rules_cc might be so potential users understand whether they should be using this or `@bazel_tools//cpp` rules if they want to integrate with the Bazel starlark rules for C/C++