Add link(s) to mailing list

[melange396]

We dont have an obvious link to our "API" mailing list on our main public website.

The only references that we have to it appear to be buried in our github pages API documentation, or buried in the covidcast R package, or indirectly mentioned in more github pages docs that make use of the covidcast R package (1, 2).

The mailing list that i would say we consider to be our current "main" for users is at https://lists.andrew.cmu.edu/mailman/listinfo/delphi-covidcast-api . We may also want to rename/rebrand this list or possibly even change to a new one if we can't get "covidcast" out of the name.