Mark plugins as experimental for now.

CHANGES.txt

@@ -20,6 +20,8 @@
     code generators.  Plugins are just binaries which accept a protocol buffer
     on stdin and write a protocol buffer to stdout, so they may be written in
     any language.  See src/google/protobuf/compiler/plugin.proto.
+    **WARNING**:  Plugins are experimental.  The interface may change in a
+    future version.
   * inf, -inf, and nan can now be used as default values for float and double
     fields.
 

src/google/protobuf/compiler/code_generator.h

@@ -107,6 +107,8 @@ class LIBPROTOC_EXPORT OutputDirectory {
   // at the given insertion point.  See plugin.proto for more information on
   // insertion points.  The default implementation assert-fails -- it exists
   // only for backwards-compatibility.
+  //
+  // WARNING:  This feature is currently EXPERIMENTAL and is subject to change.
   virtual io::ZeroCopyOutputStream* OpenForInsert(
       const string& filename, const string& insertion_point);
 

src/google/protobuf/compiler/plugin.proto

@@ -30,6 +30,9 @@
 
 // Author: kenton@google.com (Kenton Varda)
 //
+// WARNING:  The plugin interface is currently EXPERIMENTAL and is subject to
+//   change.
+//
 // protoc (aka the Protocol Compiler) can be extended via plugins.  A plugin is
 // just a program that reads a CodeGeneratorRequest from stdin and writes a
 // CodeGeneratorResponse to stdout.