Distinguish rtmidi-python from pyrtmidi

[superquadratic]

Add a README paragraph explaining the differences to pyrtmidi.

[josephernest]

Oh I just noticed these are 2 different modules !
(During a few hours, I thought it was a single project...)

[diegoherranz]

Let me add another almost-identically-named package: python-rtmidi.

It's not trivial to choose one.