update contribution policy to mention "fake" module files for missing software installations

[boegel]

We have some cases where we can't install specific modules with reasonable effort for specific CPU targets (like modules using a 2022b toolchain for zen4, cfr #874).

The contribution policy currently states:

Software that is added to EESSI should work on all [supported CPU targets](https://www.eessi.io/docs/software_layer/cpu_targets/).

Exceptions to this requirement are allowed if technical problems that can not be resolved with reasonable effort prevent the installation of the software for specific CPU targets.

This should be extended a bit to mention the "fake" module files that produce a useful error message when they're not actually providing access to an existing software installation