add pypi instructions

[graingert]

No description provided.

[lantz]

Hi @graingert - I've thought of having a mininet pypi package for some time, but it seems to me that we would need a python version of mnexec to make it pure python (though it would still depend on linux, at least until we merge in the BSD support etc..)

Thanks also for stepping forward to help make that happen (I think? I should really look at the pypi package as I had not seen it until now.)

I think we should probably merge any pypi packaging into the main repo and push to it pypi based on the CI on github. Do you have a PR for one or both of those things? Drop me a line if you would like to make that happen.