Add note for debugging the backend

Just in case I lose my terminal history, I'm saving a curl command used
to test the backend.

back_end/notes.md

@@ -0,0 +1,5 @@
+Use something like:
+
+    curl -X POST 127.0.0.1:1237 -H 'Content-Type: application/json' -d '{"type": "pairing_request"}'
+
+To debug the backend. See protocol specifications for what should work.