adding changelog

Author: mansona

CHANGELOG.md

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+### 2.2.0
+- Remove need for consuming app to configure `ember-cli-bootstrap` addon #87 @ppcano
+- Refer to Discord vs. Slack Fixes #85 @sivakumar-kailasam
+- updated the blog header component to remove unnecessary complexity #81 @MelSumner
+- list component refactor, added contributor invite #80 @MelSumner
+- updated the button component and supporting documentation #79 @MelSumner
+- Use ember-cli-addon-docs for dummy app #76 @hakilebara