FEP-67ff: FEDERATION.md

https://codeberg.org/fediverse/fep/src/branch/main/fep/67ff/fep-67ff.md

Yet another Fediverse Enhancement Proposal. FEDERATION.md file is used by some projects to document the federation protocol.

Here's the one I wrote for #Mitra: https://codeberg.org/silverpill/mitra/src/branch/main/FEDERATION.md