Added validate_edp method to Plugin SPI doc

Change-Id: Ie14b914e15139396740573daf5b7a19592145d47
Closes-Bug: #1316219

doc/source/devref/plugin.spi.rst

@@ -45,14 +45,22 @@ Lists tags, that should be added to OpenStack Image via Image Registry. Tags are
 *Example return value*: ["tag1", "some_other_tag", ...]
 
 validate(cluster)
-~~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~
 
 Validates a given cluster object. Raises *SaharaException* with meaningful message.
 
 *Returns*: None
 
 *Example exception*: <NotSingleNameNodeException {code='NOT_SINGLE_NAME_NODE', message='Hadoop cluster should contain only 1 NameNode instance. Actual NN count is 2' }>
 
+validate_edp(cluster)
+~~~~~~~~~~~~~~~~~~~~~
+
+Validates that given cluster can be used to run EDP jobs. In case of
+incompatibility raises *SaharaException* with meaningful message.
+
+*Returns*: None
+
 validate_scaling(cluster, existing, additional)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~