Prepare changelog for v3.15.0

Author: RiotRobot

CHANGELOG.md

@@ -1,3 +1,19 @@
+Changes in [3.15.0](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v3.15.0) (2021-03-01)
+=====================================================================================================
+[Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v3.15.0-rc.1...v3.15.0)
+
+## Security notice
+
+matrix-react-sdk 3.15.0 fixes a low severity issue (CVE-2021-21320) where the
+user content sandbox can be abused to trick users into opening unexpected
+documents. The content is opened with a `blob` origin that cannot access Matrix
+user data, so messages and secrets are not at risk. Thanks to @keerok for
+responsibly disclosing this via Matrix's Security Disclosure Policy.
+
+## All changes
+
+ * Upgrade to JS SDK 9.8.0
+
 Changes in [3.15.0-rc.1](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v3.15.0-rc.1) (2021-02-24)
 ===============================================================================================================
 [Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v3.14.0...v3.15.0-rc.1)