Recognize README.md as well as index.md for index files

[ryan-blunden]

Dociste shouldn't force developers to use index.md instead of README.md files. READMD.md files are advantageous because GitHub, for example, will render the markdown when viewing files at the directory level if a README.md file exists.

Having docsite support both README.md and index.md files would improve user experience when browsing docs code on a code host.