Hi Anirudh,
That's great that you want to help. All of the documentation is in the Sphinx docs, which you can view either in the source files (the comments at the begginging of classes and methods), online [1] (which may be out of date) or you can build the Sphinx docs yourself [2]. There are also a couple example notebooks in examples/notebooks you can look at that do a couple things with quantum. For getting started, the best thing to do is probably to play with it, look at the examples in the docs and try to do something, take a look at the code and see what you can do with it. There are a couple open issues for the physics module [3], but there isn't really much there to get started. If while you're playing around you have any questions, you can ask on this google group and someone will try to help you.
As a more general comment about contibuting (not specific to the physics module), you should take a look at the development workflow on the github wiki [4]. That details how to setup your git repository and how we handle merging commits into the master branch. If you have any questions on this or run into any problems, again, you can ask the google group.
Do you have anything in mind for what you'd want to do? If you had any more specifics on your physics knowledge or what you want to work with, I could probably give you some more direction.
Sean
[1]
http://docs.sympy.org/dev/index.html
[2]
https://github.com/sympy/sympy/blob/master/doc/README.rst[3]
http://code.google.com/p/sympy/issues/list?q=label%3APhysics
[4]
https://github.com/sympy/sympy/wiki/Development-workflow