Add Folly's implementation of T-Digest to README

Folly's implementation of t-digests is lightning fast and easy to use.

* Easy to use APIs for both single digest aggregation and sliding window aggregation
* It's cpu-local buffering enables 60ns writes across threads.
* It's merge algorithm leverages std::inplace_merge to speed up digest aggregation (across sliding windows, or when merging cpu-local buffers)
* It's smart about it's choice of k function to avoid expensive floating point operations
* Well tested, with benchmarks and benchmark results included in the results.
* Used in production at Facebook.

README.md

@@ -85,7 +85,7 @@ The t-digest algorithm has been ported to other languages:
  - Python: [tdigest](https://github.com/CamDavidsonPilon/tdigest)
  - Go: [github.com/spenczar/tdigest](https://github.com/spenczar/tdigest)
  - Javascript: [tdigest](https://github.com/welch/tdigest)
- - C++: [CPP TDigest](https://github.com/gpichot/cpp-tdigest)
+ - C++: [CPP TDigest](https://github.com/gpichot/cpp-tdigest), [FB's Folly Implementation (high performance)](https://github.com/facebook/folly/blob/master/folly/stats/QuantileEstimator.h)
  - Scala: need link!
 
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