docs: document how to work with config.js

Author: freaktechnik

CONTRIBUTING.md

@@ -14,6 +14,19 @@ mode with live reloading with `npm start`.
 
 There are some linters and tests. `npm test` runs all linters and tests.
 
+### Updating config.js
+
+To avoid comitting production values in `config.js`, make sure to ignore it with
+```bash
+git update-index --assume-unchanged scripts/config.js
+```
+
+If you want to commit a change to the checked in version, you can use the following to
+track changes again:
+```bash
+git update-index --no-assume-unchanged scripts/config.js
+```
+
 ### License
 All code should be licensed under the [MPL-2.0](LICENSE). By submitting a pull
 request you agree that your code is licensed that way.