document new vectorized group features

[piccolbo]

https://github.com/RevolutionAnalytics/plyrmr/blob/master/docs/fast-aggregation.md

Problem is is this sufficient?

[piccolbo]

And the answer is, it's a passable design document but not very appropriate as a user level document. That should probably be part of the help() material.

[piccolbo]

There's three parts to this:

1. the problem: small reduce groups
2. the easy solution: dplyr extension, for the simple cases
3. the general solution: provide a vectorized reduce function, which is not called this way in plyrmr but you know what I mean