If your project doesn't have README.md, GitHub may parse README.rdoc to display details. If it has both, it will use README.md, silently ignoring rdoc.
Describe briefly about your project. You may also provide project's website link, badges, community & contact info (i.e. email, social site).
Runnable file (executable or minified or installation file) link. There can be links to previous versions too.
How your work can be used. It may include the prerequisites, settings, third party libraries, usage, cautions, etc.
It may include code sample, gif file, video link, or even screen shots.
Author names, contact info, etc.
List of people or community helped and inspired throughout the project
Instructions to contribute (i.e. add feature, report bug, submit patch) to the project. May include documentation link too.
Give a short intro over your license. You can give a link to the license site too.