FEP-67ff: FEDERATION.md

[silverpill]

@arcanicanis started a new topic to discuss machine-readable implementation reports

Extension Support Discovery

With inspiration of Service Discovery from XMPP (XEP-0030), which enables XMPP servers/clients to add, support, and deprecate extensions in a piecemeal manner, it may be worth implementing similar concepts within ActivityPub applications. Meanwhile, some existing applications announce details of their support and configuration in their Nodeinfo metadata. Whereas Pleroma (and derivatives), enumerates a clear list under “features” and other service limits, and PeerTube announces what functionalit…