Update documentation on installing pre-release

[adam-sweet]

** Location where change is recommended **
In the main documentation page, https://nasa.github.io/progpy/

** Recommended change **
Update the way to install a pre-release, aka a local dev version of progpy. Use whatever the recommended way you choose, I'm a noob at this. But "python3 -m pip install . " seemed to work for me.

** Reason **
I got an error message when trying to use 'pip install -e .' :

ERROR: File "setup.py" not found. Directory cannot be installed in editable mode: /home/dev/prog/progpy
(A "pyproject.toml" file was found, but editable mode currently requires a setup.py based build.)