Document requirements

[karlwessel]

Document the requirements needed for mplopengl to work. This includes

• minimal version needed for all libraries and python
• latest version tested to work for all libraries and python
• hardware requirements?
• tested operating systems

Put all that information into the readme.md

[karlwessel]

This is almost solved by PR #12. Only testing of older python versions is needed which is probably easiest by using anaconda.