tdigest-0.2.1: CHANGELOG.md

## 0.2.1

- Add size, valid, validate, and debugPrint for NonEmpty
  [#26](https://github.com/futurice/haskell-tdigest/pull/26)

## 0.2

- Add `Data.TDigest.Vector` module.

## 0.1

- Add `validateHistogram` and `debugPrint`
- Fix a pointy centroid bug.
- Add `Data.TDigest.NonEmpty` module
- Add `mean`, `variance`, `stddev`