[alias spec] Add Usage section

Summary: Introduce the ".xec" extension to be used for eCash Aliases

Test Plan: Read it and make sure it makes sense

Reviewers: bytesofman, #bitcoin_abc, Fabien, aimal

Reviewed By: #bitcoin_abc, Fabien, aimal

Subscribers: aimal

Differential Revision: https://reviews.bitcoinabc.org/D13868