[RFE] Docs need improvement how to actually build/test sysexts

[fhemberger]

Current situation

Currently the README doesn't contain a description of the scripts contained in the repository and the necessary steps to build and test a single sysext or the complete set.

Impact

This situation adds an additional hurdle for submitting PRs for existing sysexts or creating new ones.

Ideal future situation

A section in the README listing the necessary prerequitises (environment, required packages/dependencies, etc.) and build examples along with a short description of the various scripts provided in the repository.

It should also include an example how to test and verify the function of a sysext (should also be a part of the release workflow).