Add warning to module docstring

Author: odow

Diff for: src/Nonlinear/Nonlinear.jl

@@ -4,6 +4,14 @@
 # Use of this source code is governed by an MIT-style license that can be found
 # in the LICENSE.md file or at https://opensource.org/licenses/MIT.
 
+"""
+    Nonlinear
+
+!!! warning
+    The Nonlinear submodule is experimental. Until this message is removed,
+    breaking changes may be introduced in any minor or patch release of
+    MathOptInterface.
+"""
 module Nonlinear
 
 import Base.Meta: isexpr