Add a link from the repo to the running example

[kwalrath]

The README.md files (e.g. structural-directives's README) currently say something like this:

Welcome to the example application used in angular.io/dart's
structural-directives page.

To run your own copy:

I'd like to add a prominent link to where they can find the already built version. Something like this:

Welcome to the example application used in angular.io/dart's
structural-directives page.
To run this app, go to http://angular-examples.github.io/structural-directives.

To get the source code and run your own copy:

[chalin]

Closed by #13