ADD: Markdown file to keep track of external datasets of interest

[jrobrien91]

There is a request from the City-Scale thematic group to have a location that displays external datasets of interest within the CROCUS domain.

While that may ultimately be more appropriate for official CROCUS website (https://crocus-urban.org/), this list will be helpful to:

• Keep Track of Data sources for inter-comparison with CROCUS observations
• Have a list for SULI interns to review and create cookbooks for if needed
• centralized location to combine datasets we find

[mgrover1]

Go ahead and merge when you are ready!

[jrobrien91]

@mgrover1 don't have write access to this repo yet!