Adding a README for computed property example.

@kwalrath @shailen 

Please review.
Feedback incorporated. thx

On Wed, Sep 17, 2014 at 12:50 PM, Shailen Tuli <notifications@github.com>
wrote:

> In web/basics/using_a_computed_property/README.md:
>
> > @@ -1,3 +1,43 @@
> > +
> >  # Using a Computed Property
>
> Yes, let's use sentence case.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/dart-lang/polymer-dart-snippets/pull/8/files#r17689065>
> .
>