Prepare changelog for v0.12.0

Author: lukebarnard1

CHANGELOG.md

@@ -1,3 +1,11 @@
+Changes in [0.12.0](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.0) (2018-04-11)
+=====================================================================================================
+[Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.0-rc.7...v0.12.0)
+
+ * Further improve group joining/leaving feedback
+  [\#1832](https://github.com/matrix-org/matrix-react-sdk/pull/1832)
+ * Cosmetic changes to Communities button
+
 Changes in [0.12.0-rc.7](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.0-rc.7) (2018-04-10)
 ===============================================================================================================
 [Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.0-rc.6...v0.12.0-rc.7)