Clarify where repository rules should be defined

[mboes]

Description of the feature request:

Add wording in the Bazel rule deployment guidelines clarifying where repository rules should be defined: defs.bzl, repositories.bzl or elsewhere?

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

Currently, the documentation says "rules" should be exposed in defs.bzl. There is a section titled "Repository rules", which one would expect says what to do about repository rules, but it only discusses introducing a single macro that defines all necessary dependencies. If I have a set of rules called rules_foo, which perhaps has a few build rules and several repository rules, it's unclear where the user should expect to find all the repository rules.