Tip about environment variables (#30)

docs/frontend/for_developers.md

@@ -64,6 +64,8 @@ Additionally, the main NGINX proxy server listens on port **5000** for plain HTT
 - `REACT_APP_ALT_AUTH` - if set to `plg`, app will use plgrid authentication
 - `REACT_APP_DEPLOYMENT` - if set to `dev`, configuration wil be editable from the browser console. For example, you can change the backend URL by typing `window.BACKEND_URL="http://mynew.url"` in the browser console.
 
+**It might happen that the Node will cache values for above variables and changes in .env file won't take effect. Close frontend server if it's running, open new terminal and enter into node interactive envirionment by executing `node` command. Then type process.env.NAME_OF_VARIABLE. If it's defined run `delete process.env.NAME_OF_VARIABLE`.**
+
 ### Useful commands
 
 In order to easy configure the app, `cross-env` package for setting env is used with custom npm scripts.