User facing documentation

[andreww]

Could you add some documentation to a directory called /docs. If these are formatted in markdown it would be easy to host these using github pages, and readthedocs also knows how to display markdown.

The basic documentation ought to include:

• A short outline of what pyDis does.
• Some words on installation.
• Details of the input file format and any command line options for _pn_control.py, _atomic_control.py, _segregation_control.py, etc. I think for this I would have a short general overview of the format, then a list of options for each file type.

There are probably more things to add, but getting this started will help. I'll add anything else I think of below.