Also suggest rubocop-rspec for those using rspec-rails

[pirj]

rubocop-rspec_rails provides rspec-rails specific cops, but generic rspec cops still apply, as rspec-rails depends on rspec (core, expectations and mocks)

This partially reverts #12813

Also see rubocop/rubocop-rspec_rails#28

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[pirj]

Cc @ydah @bquorning

[bquorning]

👍🏼 but of course squash it into 1 commit.

[bbatsov]

I've squashed the commits myself. Thanks!

[ydah]

Thank you!!