Explain the purpose of the sample app in the contributors page

[rocboronat]

In #454, @IanArb invested time and effort to fix the CameraActivity, a screen in the sample app.

Those screens should be as simple as possible, and they are there just to test Barista works as expected. So they don't need to be user friendly or anything like that. In this very example, the app crashes if the user taps the take picture button, but that's not bad, because it helps us develop Barista via TDD.

We should explain it in the contributors page.