Add CHANGELOG entry

Author: pcapriotti

changelog.d/1-api-changes/mls-client-api

@@ -0,0 +1,13 @@
+The client JSON object now has an additional field `mls_public_keys`, containing an object mapping signature schemes to public keys, e.g.
+```
+{
+  ...
+  "mls_public_keys": { "ed25519": "GY+t1EQu0Zsm0r/zrm6zz9UpjPcAPyT5i8L1iaY3ypM=" }
+  ...
+}
+```
+At the moment, `ed25519` is the only supported signature scheme, corresponding to MLS ciphersuite 1.
+
+When creating a new client with `POST /clients`, the field `mls_public_keys` can be set, and the corresponding public keys are bound to the device identity on the backend, and will be used to veriy uploaded key packages with a matching signature scheme.
+
+When updating a client with `PUT /clients/:client`, the field `mls_public_keys` can also be set, with a similar effect. If a given signature scheme already has a public key set for that device, the request will fail.