Add convert_meta_variables to developers guide

Author: bvdmitri

docs/src/developers_guide.md

@@ -196,10 +196,11 @@ GraphPPL.apply_pipeline
 GraphPPL.options_vector_to_named_tuple
 GraphPPL.get_created_by
 GraphPPL.convert_to_anonymous
+GraphPPL.convert_meta_variables
 ```
 
 ## Auxiliary functionality 
 
 ```@docs 
 GraphPPL.prune!
-```
+```