[contributing] added the link for the BC promise article

Travis is reporting errors for every documentation build:

![travis_error](https://cloud.githubusercontent.com/assets/73419/2558077/45493920-b730-11e3-99d3-17fe654b3998.png)

Apparently, this error is caused because the `contributing/community/releases.rst` file links to the BC Promise article but doesn't define the link.