Adding doc paragraph on pipelining

metalibm_core/doc.dox

@@ -20,3 +20,8 @@ Folding:
 Dynamic attribute registration
   By using the static method Attributes.add_dyn_attribute which take an object inheriting from AttributeCtor it is possible to add other attributes to every Operation node with user-defined configuration (default value, required, ...). By keeping an handle on the AttributeCtor object, the attribute may even be modified between new node creation.
 
+
+Pipeline:
+  Auto pipelining uses several mechanism
+  CodeEntity registers 2 dynamic attributes, init_stage and init_op. Each time an operation is declared, it store the current values of init_stage into its own field. 
+  init_stage is initialized to 0 and update by add_pipeline_stage function