Add compute 2.10 microversion tests info in doc

Compute 2.10 microversion test were implemented in
I5097a5669d0f3b7d9016d98da0fbb58a682cef8e.

Adding the info in microversion doc.

Change-Id: Id20b47012d77711a01803ba191fc8f343644277e
diff --git a/doc/source/microversion_testing.rst b/doc/source/microversion_testing.rst
index 63ec04b..fc05b12 100644
--- a/doc/source/microversion_testing.rst
+++ b/doc/source/microversion_testing.rst
@@ -205,3 +205,7 @@
  * `2.2`_
 
  .. _2.2: http://docs.openstack.org/developer/nova/api_microversion_history.html#id2
+
+ * `2.10`_
+
+ .. _2.10: http://docs.openstack.org/developer/nova/api_microversion_history.html#id9