Add issue/doc tracking sp-descriptors

[yashrajd]

AFAICT folks are working on descriptors for silent payments.

This is needed to replace footgun-ny seedphrases and capture important, relevant info that seephrases do not.

[macgyver13]

Go forward plan is to store necessary meta data required for easing silent payments wallet recovery using BIP0329.

Building a reference implementation would be nice to work through specifics and recommend to other wallet devs

[macgyver13]

Closing due to sp output descriptor bip proposal