Add info about identity proofs to FEDERATION.md

FEDERATION.md

@@ -1,6 +1,6 @@
 # ActivityPub federation in Mitra
 
-Mitra largely follows the [ActivityPub](https://www.w3.org/TR/activitypub/) server-to-server specification but it makes uses of some non-standard extensions, some of which are required for interacting with it at all.
+Mitra largely follows the [ActivityPub](https://www.w3.org/TR/activitypub/) server-to-server specification but it makes uses of some non-standard extensions, some of which are required for interacting with it.
 
 The following activities are supported:
 
@@ -22,3 +22,31 @@ And these additional standards:
 - [NodeInfo](https://nodeinfo.diaspora.software/)
 - [WebFinger](https://webfinger.net/)
 
+Activities are implemented in way that is compatible with Pleroma, Mastodon and other popular ActivityPub servers.
+
+## Profile extensions
+
+### Cryptocurrency addresses
+
+Cryptocurrency addresses are represented as `PropertyValue` attachments where `name` attribute is a currency symbol prefixed with `$`:
+
+```json
+{
+  "name": "$XMR",
+  "type": "PropertyValue",
+  "value": "8Ahza5RM4JQgtdqvpcF1U628NN5Q87eryXQad3Fy581YWTZU8o3EMbtScuioQZSkyNNEEE1Lkj2cSbG4VnVYCW5L1N4os5p"
+}
+```
+
+### Identity proofs
+
+Identity proofs are represented as attachments of `IdentityProof` type:
+
+```json
+{
+  "name": "<did>",
+  "type": "IdentityProof",
+  "signatureAlgorithm": "<proof-type>",
+  "signatureValue": "<proof>"
+}
+```