Add dust tx authentication prefix to standards doc

Summary:
Adds the lokad id for dust tx authentications.
Usage:
<lokad> <utf8 identifier>

Test Plan: plonk into https://jbt.github.io/markdown-editor/ and proof read

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16625