Description

Supersedes #3664

Let's include README.md files in the render for p/ & r/. Here are the cases:

• For p/, if a README.md exists, just render it on the top of the list of files.
• For r/, there's a few cases:
  • No Render() & no README.md file in the realm - display error page
  • Render() & no README.md - just like now
  • No Render() & README.md - fallback to README.md
  • If both are present, call Render(). On the source page, if the user selects the README.md ($source&file=README.md) - render the README with an optional raw/render button, like on GitHub.

cc @gfanton @moul @alexiscolin