Feature/fix doc links

@brutski Please review and merge.

This PR fixes links in the README that must use YARD {file: ... } format because relative links need to inject 'file' in the path per YARD generation of files in 'doc' folder.
Hmmm... I could be wrong... but it doesn't appear that this link is rendering correctly in the output.
It is showing up as regular text, thus appears like as is... ``` {file:doc/api_descriptor_documents.md API descriptor document} ```