Document CamelCase for Python class names

PEP8 calls for CamelCase class names [1], and we already do that in
the existing Python code. So we document that Python class names are
expected to come in CamelCase.

[1] “Class names should normally use the CapWords convention.”
(from http://legacy.python.org/dev/peps/pep-0008/#class-names )

Change-Id: I98e0c0bc0cb8846fa73501a36b606bfc4bf139bd