Define Registry

[joshmoore]

From the 2021-05-05 community meeting, there was a call for defining in this repository an official list (incl. machine parseable version) of all known codecs.

Third-parties can open PRs to register their own codec names to prevent collisions.

cc'ing known codec implementors: @cgohlke @jakirkham @d-v-b
and known interested parties: @DennisHeimbigner @axtimwalde

[joshmoore]

As a complete aside, I ran into this table for a self-described self-describing representation: https://github.com/multiformats/multicodec/blob/master/table.csv