Prepare changelog for v0.13.1

Author: dbkr

CHANGELOG.md

@@ -1,3 +1,9 @@
+Changes in [0.13.1](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.13.1) (2018-08-20)
+=====================================================================================================
+[Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.13.1-rc.1...v0.13.1)
+
+ * No changes since rc.1
+
 Changes in [0.13.1-rc.1](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.13.1-rc.1) (2018-08-16)
 ===============================================================================================================
 [Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.13.0...v0.13.1-rc.1)