Added comment how to get list of network extensions in tempest.conf

Change-Id: I1f0d819e0f9a73d0477a3665a9e6fc4394f69f72
Closes-bug: #1428942
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index b3aca42..467c5aa 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -792,7 +792,8 @@
 
 # A list of enabled network extensions with a special entry all which
 # indicates every extension is enabled. Empty list indicates all
-# extensions are disabled (list value)
+# extensions are disabled. To get the list of extensions run: 'neutron
+# ext-list' (list value)
 #api_extensions = all
 
 # Allow the execution of IPv6 subnet tests that use the extended IPv6