Prepare changelog for v3.18.0

Author: RiotRobot

CHANGELOG.md

@@ -1,3 +1,11 @@
+Changes in [3.18.0](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v3.18.0) (2021-04-12)
+=====================================================================================================
+[Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v3.18.0-rc.1...v3.18.0)
+
+ * Upgrade to JS SDK 9.11.0
+ * [Release] Tweak appearance of invite reason
+   [\#5848](https://github.com/matrix-org/matrix-react-sdk/pull/5848)
+
 Changes in [3.18.0-rc.1](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v3.18.0-rc.1) (2021-04-07)
 ===============================================================================================================
 [Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v3.17.0...v3.18.0-rc.1)