README.md => README.rst for better PyPI page

Preview the rendered result at https://github.com/msabramo/pytest-httpbin/tree/README.md_to_README.rst

I think it looks pretty good.

Alternatively, you could keep the `README.md` in Markdown and use some voodoo with [pypandoc](https://github.com/bebraw/pypandoc) in `setup.py` to convert on the fly. But I figured a simple static conversion is less likely to have weird problems.