Unify README with PYPI

[webknjaz]

I think it's time to:

• turn README into README.rst
• add https://github.com/yaml/pyyaml/blob/master/setup.py#L5-L16 there
• read README.rst in setup.py, so that it will match https://pypi.org/p/PyYAML and look nicer
• add badges, like PYPI, Travis/AppVeyor, so that users would be able to find places on the Internet where actual tests are being run

[penguinolog]

It's good idea to add additional links at setup.cfg:

[metadata]
project_urls =
    Bug Tracker = ...
    Documentation = ...

[webknjaz]

yep