An idea for helping humans and machines understand when a twtxt-feed are using some of the yarn.social extensions by applying a mix of a hash-bang and a DTD to the first line of the txt-file:

```#! DOCTYPE = twtxt (https://twtxt.readthedocs.io) with extension for multi-line and conversation treading from Yarn.social (https://dev.twtxt.net)```

It would be nice if the link to the extension could be to `dev.yarn.social`, `docs.yarn.social` or similar instead of `dev.twtxt.net` to make it less confusing.