Create README for this repo.

[demitri]

I'd like to request a detailed README be written for this repo. This is a great project that should be documented and advertised as such, and GitHub READMEs have become the first port of call for these kinds of things. Suggestions for inclusion:

• features and functionality of AST
• some history showing how long it's been developed
• projects/surveys that use AST
• a note on the funding level compared to its impact
• directions on installation, specific to the C version here
• pointers to other versions
• link to documentation
• issues to be aware of for people who use the code (see ast.h subject to collision with python's ast.h in some build processes #6)
• pretty pictures

I'm a big fan of this library and would like to see its visibility increase. Thanks to all for the work on it!

[dsberry]

I'll add this to me do list. It may not happen immediately due to pressure of other work, but I'll get back to it.