Improved intersection error note in README.

Just wrote a [blog post](http://blog.aggregateknowledge.com/2012/12/17/hll-intersections-2/) on  intersection error in HLLs and then came across your implementation. Thought it might help to have a slightly more elaborate description of the factors contributing to the intersection error.

One really important thing to emphasize is that intersection error grows explosively when intersecting more than three sets. The reason is fairly straightforward: the intersection cardinality is monotonically decreasing and the number of terms (and hence propagated error) in the inclusion-exclusion formula grows at least exponentially in the number of sets.

Cheers!