Add developer docs for the python client

I am far from a Python expert. Especially with Cython in the mix, so it took me a bit just to get started working on the kudu python client. 

We should document basic steps for how to develop and test the kudu python client. Including environment setup, building, and testing (running a single test too).

For now I essentially boiled my work down to this:

cd /path/to/kudu
cd build/debug 
make -j4
make install
cd /path/to/kudu/python
git clean -fdx
export KUDU_HOME=/path/to/kudu
pip install -r requirements.txt
python setup.py build_ext
python setup.py test