Prepare changelog for v0.12.0-rc.3

Author: dbkr

CHANGELOG.md

@@ -1,3 +1,10 @@
+Changes in [0.12.0-rc.3](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.0-rc.3) (2018-03-20)
+===============================================================================================================
+[Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.0-rc.2...v0.12.0-rc.3)
+
+ * Fix room tile badge not disappearing when receiving a read receipt
+   [\#1807](https://github.com/matrix-org/matrix-react-sdk/pull/1807)
+
 Changes in [0.12.0-rc.2](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.12.0-rc.2) (2018-03-19)
 ===============================================================================================================
 [Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.12.0-rc.1...v0.12.0-rc.2)