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 (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