Added documentation on building other libraries using generated libraries

README.md

 Protoc compiler Dart plugin
 ===========================
 
 This application provides a plugin for protoc compiler which
 generates pure Dart library to deal with protobufs.
 
 Please, do not forget that generated libraries depend on runtime
 support library which can be found [here](https://github.com/dart-lang/dart-protobuf).
 
 How to build and use
@@ skipping 46 matching lines @@
 
     --dart_out="field_name=MyMessage.has_field|HasFld:."
 
 Will generate the following message field accessors:
 
     String get hasFld => getField(1);
     void set hasFld(String v) { setField(1, v); }
     bool hasHasFld() => hasField(1);
     void clearHasFld() => clearField(1);
 
+Using protocol buffer libraries to build new libraries
+------------------------------------------------------
+
+The protocol buffer compiler produces one library for each `.proto` file
+it compiles. In some cases this is not exactly what is needed, e.g one
+would like to create new libraries which exposes the objects in these
+libraries or create new librares combining object definitions from
+several `.proto` libraries into one.
+
+The best way to aproach this is to create the new libraries needed and
+re-export the relevant protocol buffer classes.
+
+Say we have the file `m1.proto` with the following content
+
+    message M1 {
+      optional string a;
+    }
+
+and `m2.proto` containing
+
+    message M2 {
+      optional string b;
+    }
+
+Compiling these to Dart will produce two libraries in `m1.pb.dart` and
+`m2.pb.dart`. The following code shows a library M which combines
+these two protocol buffer libraries, exposes the classes `M1` and `M2` and
+adds som additional methods.
+
+    library M;
+
+    import "m1.pb.dart";
+    import "m2.pb.dart";
+
+    export "m1.pb.dart" show M1;
+    export "m2.pb.dart" show M2;
+
+    M1 createM1() => new M1();
+    M2 createM2() => new M2();
+
 Hacking
 -------
 
 Remember to run the tests. That is as easy as `make run-tests`.
 
 The default way of running the Dart protoc plugin is through the
 generated `out/protoc-gen-dart` script. However when run this way the
 Dart code is assembled into one large Dart file using dart2dart. To
 run with the actual source in the repository create an executable
 script called `protoc-gen-dart` with the following content:
@@ skipping 14 matching lines @@
     $ protoc --plugin=protoc-gen-dart=./plugin --dart_out=. test.proto
 
 Useful references
 -----------------
 
 * [Main Dart site](http://www.dartlang.org)
 * [Main protobuf site](https://code.google.com/p/protobuf)
 * [Protobuf runtime support project](https://github.com/dart-lang/dart-protobuf)
 * [DartEditor download](http://www.dartlang.org)
 * [Pub documentation](http://pub.dartlang.org/doc)