Add Cinder microversion test info in doc

Below patch implemented Cinder microversion 3.3 test case.
- I3d9b3fe288333721bf3b2c6c988949f2f253bfcc

Thsi commit adds the information about tested microversion
in doc.

Change-Id: I17e66b3732657a5606e300b504bbf474abc073fa
diff --git a/doc/source/microversion_testing.rst b/doc/source/microversion_testing.rst
index 572d425..adbd2dc 100644
--- a/doc/source/microversion_testing.rst
+++ b/doc/source/microversion_testing.rst
@@ -247,3 +247,9 @@
  * `2.42`_
 
  .. _2.42: http://docs.openstack.org/developer/nova/api_microversion_history.html#maximum-in-ocata
+
+* Volume
+
+ * `3.3`_
+
+ .. _3.3:  https://docs.openstack.org/developer/cinder/devref/api_microversion_history.html#id4