Add a link to exercism.io/hello from the home page

This is the change to address #1857 in pull request form.

Happy to iterate or tweak as needed. :smiley:

Screenshot:
![screen shot 2014-09-25 at 10 15 14 am](https://cloud.githubusercontent.com/assets/77174/4415352/7e0f942a-4529-11e4-830e-30f7f508c633.png)