Add example response to docs

docs/PROTOCOL.rst

@@ -972,6 +972,17 @@ Return concatenated block headers as hexadecimal from the main chain.
     The maximum number of headers the server will return in a single
     request.
 
+**Example Response**
+
+::
+
+  {
+      "count": 2,
+      "hex": "0100000000000000000000000000000000000000000000000000000000000000000000003ba3edfd7a7b12b27ac72c3e67768f617fc81bc3888a51323a9fb8aa4b1e5e4a29ab5f49ffff001d1dac2b7c010000006fe28c0ab6f1b372c1a6a246ae63f74f931e8365e15a089c68d6190000000000982051fd1e4ba744bbbe680e1fee14677ba1a3c3540bf7b1cdb606e857233e0e61bc6649ffff001d01e36299'"
+      "max": 2016
+  }
+
+
 mempool.get_fee_histogram
 =========================