Clarify nox

[fredjaya]

To make it more accessible for new contributors, add some additional nox information when installing cogent3 for development.

Suggestions:

• [ ] Add one sentence on the purpose of nox
• [ ] One sentence on the suggested installation method (pip? apt?)
• [x] Update the code chunk to (up until this step in the wiki, you're still in repos/c3dev) :

  cd ~/repos/cogent3
  nox -s test-3.8

• [x] Update the python versions in code chunks #35
• [x] Provide an example of a nox test passing

[fredjaya]

Key thing was to add an example of a passed nox test in the dev install how-to. Any more explanation should go in the testing-specific sections.