message: Document ES5 adapter in README.
diff: @@ -198,6 +198,23 @@ mergeStream(project.sources(), project.dependencies())
  .pipe(gulp.dest('build/'));
  ```
  +### Custom Elements ES5 Adapter
  +
  +If your build pipeline outputs ES5 custom elements (either from source or by compilation with a tool like Babel), it is critical to include the [Custom Elements ES5 Adapter](https://github.com/webcomponents/custom-elements/blob/master/src/native-shim.js). This adapter provides compatibility between custom elements defined as ES5-style classes and browsers with native implementations of the Custom Elements API, such as Chrome. See the [adapter documentation](https://github.com/webcomponents/custom-elements/blob/master/src/native-shim.js) for details of why this is necessary.
  +
  +#### project.addCustomElementsEs5Adapter()
  +
  +This method will return a transform stream that identifies your entrypoint HTML file (by looking for a webcomponents polyfill import) and injects a block of HTML that loads the Custom Elements ES5 Adapter. You can use this in a build pipeline to conditionally inject the adapter only when you output ES5 (as it is not needed when you output ES6).
  +
  +```js
  +const gulp = require('gulp');
  +const mergeStream = require('merge-stream');
  +
  +mergeStream(project.sources(), project.dependencies())
  + .pipe(project.addCustomElementsEs5Adapter())
  + .pipe(gulp.dest('build/'));
  +```
  +
  ### Multiple Builds
  