Update Style guide to reflect the current workflow (#4457)

Author: michaelosthege

CONTRIBUTING.md

@@ -145,10 +145,11 @@ docker exec -it pymc3 jupyter notebook list
 
 ## Style guide
 
-Follow [TensorFlow's style guide](https://www.tensorflow.org/community/contribute/code_style) or the [Google style guide](https://google.github.io/styleguide/pyguide.html) for writing code, which more or less follows PEP 8.
+We have configured a pre-commit hook that checks for `black`-compliant code style.
+We encourage you to configure the pre-commit hook as described in the [PyMC3 Python Code Style Wiki Page](https://github.com/pymc-devs/pymc3/wiki/PyMC3-Python-Code-Style), because it will automatically enforce the code style on your commits.
 
-For documentation strings, we *prefer* [numpy style](https://numpydoc.readthedocs.io/en/latest/format.html) to comply with the style that predominates in our upstream dependencies.
+Similarly, consult the [PyMC's Jupyter Notebook Style](https://github.com/pymc-devs/pymc3/wiki/PyMC's-Jupyter-Notebook-Style) guides for notebooks.
 
-Finally, see the [PyMC3 Python Code Style](https://github.com/pymc-devs/pymc3/wiki/PyMC3-Python-Code-Style) and the [PyMC's Jupyter Notebook Style](https://github.com/pymc-devs/pymc3/wiki/PyMC's-Jupyter-Notebook-Style) guides.
+For documentation strings, we *prefer* [numpy style](https://numpydoc.readthedocs.io/en/latest/format.html) to comply with the style that predominates in our upstream dependencies.
 
 #### This guide was derived from the [scikit-learn guide to contributing](https://github.com/scikit-learn/scikit-learn/blob/master/CONTRIBUTING.md)