Improve README.md

[brianspiering]

It would be helpful if the README had a longer description.

Something like:

A one-pass compiler for a subset of C, relying on a recursive-descent parser, doing the lexing, parsing and code generation in lockstep. The generated code, consisting of abstract machine instructions, is then executed by an instruction fetch and execute loop. https://news.ycombinator.com/item?id=22353532

[dfzlovebc]

definately