README: Doodle poll for 2nd registries call (and beyond?)

Greetings DID-WG people,

The chairs would like to arrange a second registries call.

Please fill out the Doodle poll below for your availability if you are
interested in participating in this call.

Note that we give options for both next week AND THE FOLLOWING WEEK.
Please fill out your availability for both weeks (since we may wish to
schedule another call that following week, likely on another topic such as
metadata).  We are having to redo the Doodle poll now because some people
did not fill out the poll for the second week.

The DEADLINE for filling out the poll is right before our regular DID WG
call on Tuesday so we can make an announcement during the call.

https://doodle.com/poll/sbdgmemqsein7spq

-- dan (for the Chairs)