The Rails documentation uses the `:include:` directive to inline the
README of the framework into the main documentation page. As the
README's aren't in the root directory from where SDoc is run we need to
add the framework path to the include:

    # :include: activesupport/README.md

This results in a warning when installing the gems as generating the rdoc for the gem is run from the gem/framework root:

    Couldn't find file to include 'activesupport/README.rdoc' from lib/active_support.rb

The `:include:` RDoc directive supports includes relative to the current
file as well:

    # :include: ../README.md

This makes sure it works for the Rails API docs and the separate gems.

Co-authored-by: Jonathan Hefner <jonathan@hefner.pro>