Document how to build a debug version of robotpy packages

[virtuald]

git clone <robotpy-repo>
pip install robotpy-build wheel
export RPYBUILD_DEBUG=1
export RPYBUILD_PARALLEL=1
python setup.py bdist_wheel
pip install dist/whatever.whl

[virtuald]

Also should document when this is a useful thing to do (most of the time it isn't). See robotpy/robotpy-commands-v2#21