Support README.md in git repos

Cgit has the ability to render README files a la github. The config option is readme=[branch]:readme, so

readme=master:README.md

in /etc/cgitrc will render display the README file from the master branch. Note that this file is not pre-processed in anyway so it'll have to be plain text. This option can be specified multiple times to support different filename choices.

To support markdown, the about-filter option will also need to be set. The supplied /usr/lib/cgit/filters/about-formatting.sh supports markdown (via the markdown package). This might need some security review.

Most of this report was cogged straight from https://f-o.org.uk/2018/markdown-readme-files-in-cgit.html