add changelog

Author: mitchellhenke

CHANGELOG.md

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+# Changelog
+
+## 2.0.0
+
+* Enhancements
+  * Return a task when sending a Sentry event
+
+* Bug Fixes
+  * Ensure `mix sentry.send_test_event` finishes sending event before ending Mix task
+
+* Backward incompatible changes
+  * `Sentry.capture_exception/1` now returns a `Task` instead of `{:ok, PID}`