Small improvement to Pyomo installation instructions

[adowling2]

I always forget how to do a developer installation, so I always need to look it up.

The website, http://www.pyomo.org/installation, suggests running

git clone https://github.com/Pyomo/pyomo

and then

python setup.py install

Suggestion. Add two sentence: "Want to contribute to Pyomo? See the documentation https://pyomo.readthedocs.io/en/stable/contribution_guide.html#setting-up-your-development-environment to configure your developer environment."

[adowling2]

The two downsides:

• Slightly more cluttered.
• Link would break if the documentation was reorganized.