Improve the documentation

[ValentijnvdBeek]

Clay currently misses a few vital pieces of documentation that need to be added in the future. Preferably before the next version is released however, in my opinion, this is not a blocking issue.

The documentation that should written or modified:

• [ ] an usage man page
• [ ] a configuration man page
• [ ] sphinx documentation
• [ ] an rst version of the README for PyPy
• [ ] improved docstrings using an unified format