Add user guide.

[seanfisk]

It should explain how to use ecs.

[fishwasser]

yes

[baudren]

Since I used this library some time ago, and struggled to find the information, I agree that this is needed. I just forked the project, and will work on kickstarting the documentation, with some example usage.

Just going through the docs folder, though, the README specifies to run sphinx-apidoc to generate the needed files. However, when I do this, the make html command of sphinx does not run - because some things are documented twice. Is this README a remnant of a previous make work, @seanfisk ? Can I safely ignore it?