Documentation for everything

[Sinono3]

Right now there is zero documentation for souvlaki asides from the example. These are the things I'd like to do about it:

• [X] Add a short code snippet in the README showing the basic functionality.
• [X] Make the docs work properly with the platforms. Right now you can't access windows docs from docs.rs. Supposedly, the docs are enabled for Linux, macOS and Windows. The docs should be updated on the next crates.io version.
• [x] Add docstrings in the source code.

[Sinono3]

Documentation is a lot better now. Closing.