Add an introduction and a very simple example

The travis configuration is wrong in some way I don't understand, though.  It does the sphinx jobs on Python 2.7 and PyPy despite the matrix exclude directive.

:question: I have found it useful to include the README in the main page of documentation, since it typically includes useful info like "what is this, where do I download it" etc.. If you agree but this will require significant editing of README I suggest deferring that to next release.