Document the convention for generating an Amendment ID

[intelliot]

This would be a good place for it: https://xrpl.org/amendments.html

By convention, it is the sha512half hash of the amendment name. https://github.com/xpring-eng/xrp-ledger-countdown/blob/e773351bfb0be282bfd950652a373f294f27965d/index.js#L50