Extends README.md to mention indirect use.

reflectable/README.md

 # Reflectable
 
 *A listing of known limitations is given at the end of this page.*
 
 ## Introduction
 
 This package provides support for reflection which may be tailored to cover
 certain reflective features and omit others, thus reducing the resource
 requirements at runtime.
 
@@ skipping 121 matching lines @@
 dependencies:
   reflectable: any
 
 transformers:
 - reflectable:
     entry_points:
       - web/main.dart # The path to your main file
     formatted: true # Optional.
 ```
 
-Note that it is possible to use globbing with entry points, based on the
-package [glob](https://pub.dartlang.org/packages/glob). For instance,
-`entry_points: ["test/*_test.dart"]` can be used to specify that all Dart
-source files in `test` whose name ends in `_test.dart` are entry points.
+Note that it is necessary to include the reflectable transformer in the
+`pubspec.yaml` of your package if you use reflectable directly *or indirectly*
+(by depending on a package that uses reflectable), because the transformer may
+need to generate code based on files in your package. In particular, if you
+are writing a package which uses reflectable and is expected to be used by
+clients, please make it explicit that such clients must include the reflectable
+transformer in their `pubspec.yaml`.
+
+It is possible to use globbing with entry points, based on the package
+[glob](https://pub.dartlang.org/packages/glob). For instance, `entry_points:
+["test/*_test.dart"]` can be used to specify that all Dart source files in
+`test` whose name ends in `_test.dart` are entry points.
 
 Now run `pub build --mode=debug web` to perform the transformation. This will
 rename the file `web/main.dart` and generate a new file `build/web/main.dart`.
 That file contains the data needed for  reflection, and a main function that
 will initialize the reflection framework before running the original `main`.
 When you run this file, it is important that the package-root is set to
 `build/web/packages`, because the reflectable package itself is transformed
 to a version that uses the generated data, instead of using `dart:mirrors`.
 
 Some elements in this scenario are optional: In the `pub build..` command, you
@@ skipping 136 matching lines @@
 
 - The mirror method `libraryDependencies` has not yet been implemented with
   transformed code.
 
 ## Feature requests and bug reports
 
 Please file feature requests and bugs using the
 [github issue tracker for this repository][7].
 
 [7]: https://github.com/dart-lang/reflectable/issues