README: Doodle poll for registry/ies call

Greetings DID-WG people,

It is time for another special Topic Call!

The chairs would like to arrange a special call focused on a single topic:
"the registry".  As a reminder, "the registry" can mean one or more
registries as determined by the group in its work.

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

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/ztvun7vatamu2yz9

-- dan (for the Chairs)