Sphynx doc with usage and also API

[scls19fr]

Maybe a real doc (using Sphynx for example) should be written.

Here is an example (which probably have room for improvement)
http://windrose.readthedocs.io/en/latest/install.html
https://github.com/python-windrose/windrose/tree/master/docs

Sphynx is not limited to reStructuredText, it also accept Markdown (which is better for most contributors)

API doc is generate automatically and can be build for each push using trigger and/or Travis build.

Doc can be available at Read The Docs or as GitHub pages.
I don't know what is the best...

Anyone want to tackle this?

[scls19fr]

With doctests #89 (PR #90) API doc looks much better

[scls19fr]

#92 provides Sphynx doc.

@k-bx could you go to https://readthedocs.org/dashboard/import/ to import it.

RTD can be set to automatically build doc for every push.

[scls19fr]

A screenshot to show how doc looks like