Improve documentation for creating a parser

[gregsexton]

Document this thoroughly and provide examples.

[imdaveho]

Hi @gregsexton, really like this project as a flexible text folding package for emacs. I agree with your assessment that the documentation for creating a parser is a bit shoddy, but I think you just need to give a simple example to help others understand the what functions to call.

I simply wanted to ask how you would implement folding for both stuff between braces (c-style-parser) and brackets? If you could give an example, that would be great! Thanks.