Add PFAS data to GitHub alongside data dictionary.

[dwreeves]

In the Google Drive folder Contaminant Overviews/PFAS, there are some notes by Mia and Oscar relating to PFAS data. We have a few needs incorporating this into our repo:

• Bring Oscar's code onto GitHub. (Please credit Oscar in the commit message for this file.)
• Make sure the code can replicate and push the data into the data folder, e.g. use relative paths like ../../../data/vermont_pfas.
• Create a data dictionary for this data as a README.md file.