Document system requirements

[apjanke]

Hi, kwikteam!

Could you document the system requirements for npy-matlab in its README? I'm thinking stuff like:

• What is the minimum supported version of Matlab?
• Are any additional toolboxes required or supported?
• Is there an operating system requirement?
• Would you accept a dependency on MEX files (either the old C API or the new C++ API) or a Java library?

This would be helpful to both users, and potential contributors, so they can know what level of Matlab features they can use in their code contributions.