Rename README to README.md

[cppjames]

Rename README to README.md so that GitHub recognizes it as a Markdown file.

[DamieFC]

I think that README.txt would be better than just plain-old README because I know for me the computer treats it as an executable. And, README.txt is better than README.md because it keeps the original formatting, which Markdown doesn't 😞

[cooljeanius]

1. This is an unofficial mirror that isn't affiliated with the GCC project; pull requests here are ignored.
2. The .md file extension has a different meaning in the GCC repo (it has its own custom "machine description" format), so any addition of markdown files should come with a way of differentiating between Markdown and machine descriptions.