Add FEDERATION.md (#17029)

Some ActivityPub projects have a FEDERATION.md which is used to describe the
various extensions they use.

Everything here is also documented elsewhere, but it's a concise starting point
with links to that documentation.

FEDERATION.md

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+## ActivityPub federation in Mastodon
+
+Mastodon largely follows the ActivityPub server-to-server specification but it makes uses of some non-standard extensions, some of which are required for interacting with Mastodon at all.
+
+Supported vocabulary: https://docs.joinmastodon.org/spec/activitypub/
+
+### Required extensions
+
+#### Webfinger
+
+In Mastodon, users are identified by a `username` and `domain` pair (e.g., `Gargron@mastodon.social`).
+This is used both for discovery and for unambiguously mentioning users across the fediverse. Furthermore, this is part of Mastodon's database design from its very beginnings.
+
+As a result, Mastodon requires that each ActivityPub actor uniquely maps back to an `acct:` URI that can be resolved via WebFinger.
+
+More information and examples are available at: https://docs.joinmastodon.org/spec/webfinger/
+
+#### HTTP Signatures
+
+In order to authenticate activities, Mastodon relies on HTTP Signatures, signing every `POST` and `GET` request to other ActivityPub implementations on behalf of the user authoring an activity (for `POST` requests) or an actor representing the Mastodon server itself (for most `GET` requests).
+
+Mastodon requires all `POST` requests to be signed, and MAY require `GET` requests to be signed, depending on the configuration of the Mastodon server.
+
+More information on HTTP Signatures, as well as examples, can be found here: https://docs.joinmastodon.org/spec/security/#http
+
+### Optional extensions
+
+- Linked-Data Signatures: https://docs.joinmastodon.org/spec/security/#ld
+- Bearcaps: https://docs.joinmastodon.org/spec/bearcaps/
+- Followers collection synchronization: https://git.activitypub.dev/ActivityPubDev/Fediverse-Enhancement-Proposals/src/branch/main/feps/fep-8fcf.md