User documentation for PlayCover.

How to contribute to PlayBook properly:

1. Install rust, brew install rust.
2. Download and extract the modified build of mdbook from here.
3. Download the contents of the PlayBook repository, git clone https://github.com/PlayCover/PlayBook
4. Change the active directory to ../PlayBook/PlayBook.
5. Run <path to extracted mdbook binary> serve --open -n 127.0.0.1 to preview the book in a local web server to check your work. It also hot reloads.