Update with CLA

.github/PULL_REQUEST_TEMPLATE.md

@@ -5,4 +5,4 @@
 -   [ ] Tests written for new code (and old code if feasible).
 -   [ ] New or updated `public`/`exported` symbols have accurate [TSDoc](https://tsdoc.org/) documentation.
 -   [ ] Linter and other CI checks pass.
--   [ ] Sign-off given on the changes (see [CONTRIBUTING.md](https://github.com/matrix-org/matrix-react-sdk/blob/develop/CONTRIBUTING.md)).
+-   [ ] I have licensed the changes to Element by completing the [Contributor License Agreement (CLA)](https://cla-assistant.io/element-hq/matrix-react-sdk)

CONTRIBUTING.md

@@ -1,3 +1,5 @@
 # Contributing code to matrix-react-sdk
 
-matrix-react-sdk follows the same pattern as https://github.com/vector-im/element-web/blob/develop/CONTRIBUTING.md
+Everyone is welcome to contribute code to matrix-react-sdk, provided that they are willing to license their contributions to Element under a [Contributor License Agreement](https://cla-assistant.io/element-hq/matrix-react-sdk) (CLA). This ensures that their contribution will be made available under an OSI-approved open-source license, currently licensed under Affero General Public License v3 (AGPLv3) or General Public License v3 (GPLv3) at your choice.
+
+matrix-react-sdk follows the same pattern as element-web, please find more contributing guidelines at https://github.com/vector-im/element-web/blob/develop/CONTRIBUTING.md