minor #909 Added a warning for a:b notation usage with < 1.2 (mattkirwan)

This PR was merged into the 1.2.x-dev branch.

Discussion
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Added a warning for a:b notation usage with < 1.2

Commits
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410374fd Added a:b notation warning

doc/usage.rst

@@ -383,6 +383,10 @@ convert method will be used as converter::
         // ...
     })->convert('user', 'converter.user:convert');
 
+.. warning::
+
+    Please note that the ability to use a service method (with the `a:b` notation) will be in version 1.2
+
 Requirements
 ~~~~~~~~~~~~