Add info about Public Domain Map to the homepage

[quincylvania]

Public Domain Map lives in this tasking manager, so there should probably be a blurb on the homepage with a link out to https://www.publicdomainmap.org/. Not sure if we should wait until PDmap projects are available to the general public.

@jessbeutler Thoughts? Could you help with the write-up?

[jessbeutler]

Great point! Let's add that to the to-do list but let's not worry about having that figured out before the pilot. I'll draft up something.

[jessbeutler]

@quincylvania I'm circling back to this. Where were you thinking on the home page? That'll help me draft this out.