docs(modules_dart): add README.md

Closes angular#1628

modules_dart/README.md

@@ -0,0 +1,34 @@
+This directory contains pure Dart packages that use standard pub layout.
+
+# Working with an existing package
+
+1. Run `gulp build.dart`
+1. Open the package in any Dart IDE of your choice
+1. Tada!
+
+You do not need to rerun step 1 if you are only making changes in
+`modules_dart`. However, you do need to run it again after you make a change
+in `modules`. This is because these packages depend on code inside the `dist`
+directory via `dependency_overrides`. Code in `modules` need to be transpiled
+into `dist` before the IDE can "feel" your changes in `modules`.
+
+# Creating a new package
+
+1. Create a new directory with its own `pubspec.yaml` and standard pub layout.
+1. Add both a normal dependency on `angular2` _and_ a `dependency_overrides`,
+   like this:
+
+```yaml
+version: 0.0.0
+dependencies:
+  angular2: '0.0.0'
+dependency_overrides:
+  angular2:
+    path: ../../dist/dart/angular2
+```
+
+The publishing script will automatically rewrite version numbers, so use
+`0.0.0` both for your package and `angular2` version. Similarly, do not
+include `authors` and `homepage`, as they will be auto-populated. However,
+do provide `description`, `name` (prefixed with `angular2_`), `dependencies`,
+and `dev_dependencies`.