Prepare changelog for v3.21.0

CHANGELOG.md

@@ -1,3 +1,27 @@
+Changes in [3.21.0](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v3.21.0) (2021-05-17)
+=====================================================================================================
+[Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v3.21.0-rc.1...v3.21.0)
+
+## Security notice
+
+matrix-react-sdk 3.21.0 fixes a low severity issue (GHSA-8796-gc9j-63rv)
+related to file upload. When uploading a file, the local file preview can lead
+to execution of scripts embedded in the uploaded file, but only after several
+user interactions to open the preview in a separate tab. This only impacts the
+local user while in the process of uploading. It cannot be exploited remotely
+or by other users. Thanks to [Muhammad Zaid Ghifari](https://github.com/MR-ZHEEV) 
+for responsibly disclosing this via Matrix's Security Disclosure Policy.
+
+## All changes
+
+ * Upgrade to JS SDK 11.0.0
+ * [Release] Add missing space on beta feedback dialog
+   [\#6019](https://github.com/matrix-org/matrix-react-sdk/pull/6019)
+ * [Release] Add feedback mechanism for beta features, namely Spaces
+   [\#6013](https://github.com/matrix-org/matrix-react-sdk/pull/6013)
+ * Add feedback mechanism for beta features, namely Spaces
+   [\#6012](https://github.com/matrix-org/matrix-react-sdk/pull/6012)
+
 Changes in [3.21.0-rc.1](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v3.21.0-rc.1) (2021-05-11)
 ===============================================================================================================
 [Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v3.20.0...v3.21.0-rc.1)