Update README.markdown

Fix typo in URL
README: #haskell-lens is the place to be

I make important changes to important files.
lens 3.8 compatibility (`only` was added in 3.9)
Added a Data.Analytics.Approximate roll-up module.
Hide the _field names
Don't export the ugly versions of various field names
Import guard for lens 3.8
Committed an untested Rabin-Karp-style rolling hash based bytestring breaker.
Derp. I had swapped quot and rem.
Construct the new lazy bytestrings from the original.
2 bug fixes. first, we were giving smaller fragments because of concat. second we were resuming on the wrong bytestring.
Like a rolling hash.
Moved Data.Analytics{. => .Hash}.Rolling. Started docs for them. Refactored to move the magic into the function.
Added enhanced double hashing.