Add readthedocs

[cdeil]

@tisimst - Actually, I noticed that I can't set up http://readthedocs.org for mcerp, because it needs to set a git post-commit hook on Github, so that a new docs build there is triggered for every commit in master and tag here in Github.

Could you please create the "mcerp" project on Readthedocs and add me "deil" as maintainer in the settings? It should just take a few minutes of your time.

Once it's activated, I will configure the install of dependencies and docs build there.