Index: FEDERATION.md
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--- FEDERATION.md	(revision 162c7a7b3c290087a6741a3e1e2382de9a1d10ab)
+++ FEDERATION.md	(revision dc4aa08b9b004c449dc53ab40d77b4576e9360f4)
@@ -153,4 +153,5 @@
 ## Additional documentation
 
+- Client-to-Server API for apps (the Shaer contract): `docs/shaer-c2s-api.md`.
 - Source code: `src/services/ActivityPubService.js` (core AP logic),
   `src/routes/activitypub.js` (S2S, WebFinger, NodeInfo),