Improve README detection for non-Markdown formats

My Pod, [Relayout](https://cocoapods.org/pods/Relayout), uses a reStructuredText file for its README. This is completely valid for GitHub, as reStructuredText is one of the file formats GitHub supports for README files. However, the quality score indicator only detects Markdown-based README files, and is currently [subtracting 13 points](https://cocoapods.org/pods/Relayout/quality) from this pod due to not supporting this file type.

This is ultimately due to the ReadmeScore gem not supporting non-Markdown files. I have opened clayallsopp/readme-score#5 to address supporting other standard files.

Given that the supplied repository has a README file, and that file is just not being detected correctly by the scorer, it would be great if the quality modifiers could check this and bypass README detection in that case, rather than subtract points.

Thank you!