Markdown is the most versatile documentation.

Using the gollum ruby extension, gives you a local web-server (localhost:4567), where you can preview your documentation, before committing, and pushing to Github

There is a problem using Ruby 2.2 with gollum.  To install gollum, and resolve this issue (use Ruby 2.2 head):

rvm install ruby-2.2-head

Create a .ruby-version file (in the root of your git repository), which has a single line:

> > ruby-2.2-head > >

gem install gollum gem install github-markdown

gollum http://localhost:4567/ in a web browser

You can write, and preview your markdown, then commit and push to github (gitlab) :D