Adds section on contributing to the README.
Adds section on contributing to the README.

Contributed code must follow PEP8 rules.

By contributing to this project you agree to license your contributions under
the appropriate licenses (GNU AGPLv3 and BSD for code, CC-BY-SA 3.0 for
documentation).

donovan-graham/flask-restless,jfinkels/flask-restless,jwg4/flask-restless,jkatzer/flask-restless,emilyemorehouse/flask-restless,dan-compton/flask-restless,jkatzer/flask-restless,donovan-graham/flask-restless,ternaris/flask-restless,donovan-graham/flask-restless,dan-compton/flask-restless,dan-compton/flask-restless,mayuroks/flask-restless,quiqua/flask-restless,victorliun/flask-restless,ternaris/flask-restless,jkatzer/flask-restless,ternaris/flask-restless