Update changelog

Author: ishkawa

Changelog.md

@@ -4,6 +4,12 @@ Changelog
 Current master
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+2.1.0
+-----
+
+- Refactors internal implementation. See [#56](https://github.com/RxSwiftCommunity/Action/pull/56) and [#59](https://github.com/RxSwiftCommunity/Action/pull/59).
+- Adds SwiftPM support. See [#58](https://github.com/RxSwiftCommunity/Action/pull/58).
+
 2.0.0-beta.1
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