[yaml] Unmark as experimental

Signed-off-by: Jeffrey Kinard <jeff@thekinards.com>

sdks/python/apache_beam/yaml/README.md

@@ -44,8 +44,9 @@ or consumption (e.g. a lineage analysis tool) and expect it to be more
 easily manipulated and semantically meaningful than the Beam protos
 themselves (which concern themselves more with execution).
 
-It should be noted that everything here is still EXPERIMENTAL and subject
-to change. Feedback is welcome at dev@apache.beam.org.
+It should be noted that everything here is still under development, but any
+features already included are considered stable. Feedback is welcome at 
+dev@apache.beam.org.
 
 ## Example pipelines