Add a RethinkDB example in README

[kouak]

I ran into a few gotchas implementing dataloader with RethinkDB and I thought I'd share my workarounds.

This PR provides a RethinkDB implementation example.

Also, english is not my first language, so feel free to comment on poor phrasing :).

[coveralls]

Coverage remained the same at 100.0% when pulling 5ff6f86 on kouak:master into da2fb5f on facebook:master.

[blakeembrey]

Did you investigate the trade-offs of this approach vs the simpler r.map(keys, (key) => r.table(x).get(key))?

[leebyron]

This is pretty long vs the others in there - maybe it makes sense to set up a cookbook of examples?

@kouak, @blakeembrey what do you think about adding a folder examples/ under which we could have something like this as rethinkdb.md or rethinkdb.js