update "Contributing via CLI"

[DV-NOAA]

spoke with @underwoo about the following:

• the "Contributing via CLI" article referred to a list of Python packages, not the "requirements.txt" file
• @underwoo mentioned it would be better to have the pip3 install command reference the file
  • this allows the team to add packages to the file, without having to remember to update the article
• the instructions were also moved to a later point in the article
  • now, the user will have already "changed directory" into the cloned repo, and the path to "requirements.txt" will be correct
• also, added a GitHub link to the "requirements.txt" file, so any user can see what packages will be installed, even before cloning the repo