Add usage notice in readme file

[brettaufheber]

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[sonarcloud]

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[peterhoepfl]

The Parameter PATH_PREFIX is missing in the documentation.
Also the prefix is not reflected in the urls in the diagram.
And for the proxy additional settings may be necessary, the following list is not final:
Header always set Access-Control-Allow-Credentials "true"
        Header always set Access-Control-Allow-Headers ""
        Header always set Access-Control-Allow-Origin ""
        Header always set Access-Control-Expose-Headers "Content-Security-Policy, Location"
        Header always set Access-Control-Allow-Methods "*"
These headers were required in an test environemnt to make the preflight call of the browser to the server succeed.