README: revisit installation/usage sections

[blueyed]

Closes #376.

Preview: https://github.com/blueyed/pdbpp/blob/readme/README.rst

[Maltimore]

I think installing with pip directly from the repo is the best way, I hadn't thought of that. Two comments:

• I would put the note regarding installing directly from the repo earlier in the README (to make it more visible), for instance in the installation section above "is also available via... [conda]".
• the :program: blocks you added don't seem to get parsed correctly, at least not in the preview

[blueyed]

* I would put the note regarding installing directly from the repo earlier in the README (to make it more visible), for instance in the installation section above "is also available via... [conda]".

Good point, done.

* the `:program:` blocks you added don't seem to get parsed correctly, at least not in the preview

I've removed the usage of that role for now.
The rendering of ReST/Sphinx documents appears to be rather limited on GitHub - the plan there is to move to some more full-fledged docs, where this would make more sense then.