[doc] Add RPC response notes

doc/developer-notes.md

@@ -619,3 +619,8 @@ A few guidelines for introducing and reviewing new RPC interfaces:
   - *Rationale*: as well as complicating the implementation and interfering
     with the introduction of multi-wallet, wallet and non-wallet code should be
     separated to avoid introducing circular dependencies between code units.
+
+- Try to make the RPC response a JSON object.
+
+  - *Rationale*: If a RPC response is not a JSON object then it is harder to avoid API breakage if
+    new data in the response is needed.