Add section about platforms to rule deployment guide

### Description of the feature request:

The Bazel [rule deployment guide](https://docs.bazel.build/versions/master/skylark/deploying.html) explains what to do about constraints and where they are expected to go. The guide should likewise cover where to put `platform` definitions.

### Feature requests: what underlying problem are you trying to solve with this feature?

It is not clear where users should expect to find platform definitions. These should have a somewhat canonical and stable location, since they will be mentioned in the CLI `--host_platform` flag etc.