Add uplifting the React version in CONTRIBUTING.md

Signed-off-by: Marco Miller <marco.miller@ericsson.com>

CONTRIBUTING.md

@@ -115,6 +115,12 @@ more may be necessary depending on the case. Here is an example used to uplift f
    * `./examples/electron/package.json`
    * `./package.json`
    * `./theia-extensions/viewer-prototype/package.json`
+1. Align these React dependency versions in `./package.json` with Theia's `packages/core/package.json`, when applicable:
+   * `"react"`
+   * `"react-dom"`
+   * `"@types/react"`
+   * `"@types/react-dom"`
+1. Do the same also for `react-test-renderer` in `packages/react-components/package.json`, and potentially elsewhere in the extension.
 
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