Add mtxclient changelog

CHANGELOG.md

+# Changelog
+
+## [0.5.0] -- 2021-04-18
+
+* Support edits
+* New relation format and API
+* Fixes for voip versioning by trilene
+* Proper TLS support
+* Support for pushers by vurpo
+* Fix crashes when using the epoll reactor on ARM
+* Switch to gitlab CI
+* Support for [MSC2545](https://github.com/matrix-org/matrix-doc/pull/2545), sticker and emote packs
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