Create a Whitepaper on Self-Issued OP

[Nat Sakimura]

There seems to be a myth around Self-Issued OP such as the user is identified by URL, it needs DNS, etc. (e.g., found in ProjectVRM list)

These should be cleared out.

As it stands, since the entire picture of Self-Issued OP is scattered around in the Core spec, it probably is a bit hard for casual readers to find it out.

A whitepaper should help.

[Nat Sakimura]

Duplicate of #1027.