fix(CONTRIBUTING): remove section about beta releases (#2109)
fix(CONTRIBUTING): remove section about beta releases (#2109)

We don't have beta releases yet (see the [script](https://github.com/algolia/instantsearch.js/blob/develop/scripts/release.sh#L85-L92) and https://github.com/algolia/instantsearch.js/issues/2056), so the contributing section shouldn't be added yet.

We will put this back when -b works, and we've found a strategy for making it work with jsDeliver
algolia/instantsearch.js,algolia/instantsearch.js,algolia/instantsearch.js