README.md should be in RestructuredText format for PyPI

Created by: vtexier

Pypi display badly the README.md because it does support only RestructuredText.

https://pypi.python.org/pypi?name=duniterpy&version=0.42.3&:action=display

Github support Markdown AND RestructuredText for the documentation.

https://gist.github.com/dupuy/1855764

So we should convert the README.md to README.rst to have a good display on both platform.