If you are submitting documentation for the current stable release, submit it to the corresponding branch. For example, documentation for CoRA 1.0.0 would be submitted to the 1.0.0 branch. Documentation intended for the next release of CoRA should be submitted to the master branch.
If you are new to markdown, fear not...
Markdown is a lightweight markup language with plain text formatting syntax. It is designed so that it can be converted to HTML and many other formats using a tool.
Here are some links to help you get started