Add a README.md to bazelbuild docs folder

Add a README.md to the [docs folder](https://github.com/bazelbuild/bazel/tree/master/docs) in the `bazelbuild` repo.

The README should contain a how-to section that explains how to contribute content and fixes to the documentation, how to view content previews etc.

Start in the repository's docs folder and inspect the existing documentation and contribution or preview instructions before drafting README.md. Document how to contribute content and fixes and how to view content previews; done means the README is present in docs and covers both requested workflows accurately.

Written by the indexing model from the issue text.