Document notable changes in CHANGELOG.md.

Author: sharvil

CHANGELOG.md

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+# ChangeLog
+
+## 0.2.0 (2020-02-12)
+### Added
+- New time-fused API for LSTM (`lstm::ForwardPass::Run`, `lstm::BackwardPass::Run`).
+- Benchmarking code to evaluate the performance of an implementation.
+
+### Changed
+- Performance improvements to existing iterative LSTM API.
+- BREAKING CHANGE: `h` must not be transposed before passing it to `lstm::BackwardPass::Iterate`.
+- BREAKING CHANGE: `dv` does not need to be allocated and `v` must be passed instead to `lstm::BackwardPass::Iterate`.
+
+## 0.1.0 (2020-01-29)
+### Added
+- Initial release of Haste.