Add onSnapshot functionality to doc, collection and query

Author: BrianChapman

CONTRIBUTING.md

@@ -9,5 +9,5 @@ If you've encountered a bug or want to request new functionality, go ahead and f
 * make your additions in `./src`
 * Do not edit the files in `./browser`. They are built automatically for releases. [@bendrucker](https://github.com/bendrucker) will have to spend time rebasing your PR and that makes him :cry:.
 * add test units in `./test`
-* `npm test`
+* `npm test` (node version < 8)
 * submit a pull request when all tests pass