What file uses .md extension and how should I edit them?

Question

On GitHub, several projects have README.md files. It seems like a simple format file to express text and pictures.

I guess there is an editor or syntax

Answer 1

If you are looking for an editor, I suggest you use http://dillinger.io/. It is a simple browser-based text editor that can render Markdown on the fly.

However, if you prefer an app, and you are using OS X, you could try Mou. It is quite good and full of examples.