Documentation

I've created a very basic [Sphinx](http://sphinx-doc.org/)-based documentation, located in the doc subdirectory.

To generate the output files, cd into the doc directory and run "make html". "make" without a target gives you a list of available formats (such as epub, latex, pdf, man pages, etc).

If somebody is interested in extending the documentation, have a look at [Quick reStructuredText](http://docutils.sourceforge.net/docs/user/rst/quickref.html) and [Sphinx documentation contents](http://sphinx-doc.org/contents.html).

Once the pull request is accepted, I can set up the project on [Read the Docs](https://readthedocs.org/) so that the documentation is automatically published online.