message: Update link to matrix-js-sdk CONTRIBUTING file
diff: Contributing code to The React SDK
  ==================================
  -matrix-react-sdk follows the same pattern as https://github.com/matrix-org/matrix-js-sdk/blob/master/CONTRIBUTING.rst
  +matrix-react-sdk follows the same pattern as https://github.com/matrix-org/matrix-js-sdk/blob/master/CONTRIBUTING.md
  