update release notes

Author: FabianHofmann

doc/release_notes.rst

@@ -4,6 +4,11 @@ Release Notes
 Upcoming Version
 ----------------
 
+
+Version 0.3.8
+-------------
+
+
 * The matrices accessor of the `Model` class now has a new function `dual` which returns the dual values of the constraints if the underlying model was optimized and dual values are existent.
 
 * The Variables class now has a new function `get_solver_attribute` which parses solver-specific attributes of the variables. For now, this function only works for Gurobi `solver_model`s. For example, the function allows retrieving the variable fields `SAObjUp` or `RC`.