Add changelog

CHANGELOG.md

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+# Changelog
+
+#### Version 1.1.0 (2018-03-26)
+
+ - Improve documentation and fix some typos.
+ - Bump CI to ensure we support newer Golang versions.
+ - Add `IsEmpty()` method on `Semaphore`.
+
+#### Version 1.0.0 (2015-02-13)
+
+Initial release.