Clarify environment installation steps in CONTRIBUTING

[dcamron]

CONTRIBUTING.md had some room to catch up on our environment specification process to be more in-line with how we have it set up for CI and to more fully encompass what a user's development environment setup might look like (and preempt some potential headaches.)

• Closes Getting started information missing #1881

[kgoebber]

Is it possible to capture in this PR as well, the addition of how to install the correct matplotlib for testing images? @dopplershift and I worked through this yesterday and it could be a separate PR, but seems like it could go naturally with this one that is already started.

[dcamron]

Yeah, I'm gonna come back and flesh this out a little more proper next week. We created this as a demo exercise with our interns this morning!

[kgoebber]

I essentially used this to create a new environment to test something using Python 3.7 and this now captures the needed elements to get started. The only think I didn't have in the environment was git, which wasn't installed in my base environment, so while it is not a dependency at all, is that something we want to capture in the docs?

[dcamron]

The only think I didn't have in the environment was git, which wasn't installed in my base environment, so while it is not a dependency at all, is that something we want to capture in the docs?

If this is to serve as "from zero to dev environment" (I think so) and we expect them to clone the repository for development (we do), then absolutely! I'll add and reference something about having git installed as a prerequisite.

[dopplershift]

Found a couple things, but otherwise this looks great.