Document version request messaging protocol

docs/source/development/messaging.txt

@@ -670,6 +670,44 b' Message type: ``connect_reply``::'
     }
 
 
+Version
+-------
+
+If a client needs to know what protocol the kernel supports, it can
+ask version number of the messaging protocol supported by the kernel.
+This message can be used to fetch other core information of the
+kernel, including language (e.g., Python), language version number and
+IPython version number.
+
+Message type: ``version_request``::
+
+    content = {
+    }
+
+Message type: ``version_reply``::
+
+    content = {
+        # Version of messaging protocol (mandatory).
+        # The first integer indicates major version.  It is incremented when
+        # there is any backward incompatible change.
+        # The second integer indicates minor version.  It is incremented when
+        # there is any backward compatible change.
+        'protocol_version': [int, int],
+
+        # IPython version number (optional).
+        # Non-python kernel backend may not have this version number.
+        'ipython_version': [int, int, int, str],
+
+        # Language version number (mandatory).
+        # It is Python version number (e.g., [2, 7, 3]) for the kernel
+        # included in IPython.
+        'language_version': [int, ...],
+
+        # Programming language in which kernel is implemented (mandatory).
+        # Kernel included in IPython returns 'python'.
+        'language': str,
+    }
+
 
 Kernel shutdown
 ---------------