chore(book): adding help and contributing pages

[jeertmans]

This PR adds meta pages to help people know how they can get help and how they can contribute to this project.

The internals page is still WIP.

Some more context:

I decided to remove the "redundant" information in the README for two main reasons:

1. in my opinion, a README should just be a short introduction to the crate, especially when there exists an online documentation and a book;
2. having the same code and explanations in multiple places, but from distinct sources, makes it harder to keep up-to-date (like when you when from Iterator<Item = Token> to Iterator<Item = Result<Token, Error>>) and can easily lead to errors.