Add a link to Layout/EmptyLineAfterMagicComment

[amomchilov]

I didn't know that this was a "magic" comment, and that Layout/EmptyLineAfterMagicComment was the way to require a blank line after the frozen_string_literal comment.

I think it would be useful for the documentation to point people in the right direction.

Is there a special syntax like @see or something, for linking between these doc pages?

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). # N/A
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together. # N/A
• Added tests. # N/A
• 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. # N/A

[bbatsov]

I'm fine witht the proposed change, but there's some merge conflict you'll need to resolve before we can merge it. Down the road it'd be nice to add some "RelatedCops" metadata to cops, so it's easy to figure out this type of information.