Add CHANGELOG

CHANGELOG.md

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+# Changelog
+
+## [0.4.3] - unreleased
+
+ - Added: [[#121](https://github.com/chfast/ethash/pull/121)]
+   The public `version.h` header with information about the ethash library version.
+ - Added: [[#121](https://github.com/chfast/ethash/pull/121)]
+   Ethash and ProgPoW revisions exposed as `{ethash,progpow}::revision` constants.
+
+## [0.4.2] - 2019-01-24
+
+ - Fixed: The `progpow.cpp` file encoding changed from utf-8 to ascii.
+
+## [0.4.1] - 2018-12-14
+
+ - Added: [KISS99 PRNG](https://en.wikipedia.org/wiki/KISS_(algorithm)) distribution tester tool.
+ - Changed: ProgPoW implementation updated to revision [0.9.2][progpow-changelog].
+ - Changed: ProgPoW implementation optimizations.
+
+## [0.4.0] - 2018-12-04
+
+ - Added: Experimental support for [ProgPoW] [0.9.1][ProgPoW-changelog].
+
+
+[0.4.3]: https://github.com/chfast/ethash/compare/v0.4.2...master
+[0.4.2]: https://github.com/chfast/ethash/releases/tag/v0.4.2
+[0.4.1]: https://github.com/chfast/ethash/releases/tag/v0.4.1
+[0.4.0]: https://github.com/chfast/ethash/releases/tag/v0.4.0
+
+[ProgPoW]: https://github.com/ifdefelse/ProgPOW/blob/master/README.md
+[ProgPoW-changelog]: https://github.com/ifdefelse/ProgPOW#change-history