docs: Add link to related vorburger.exec to README

[vorburger]

@fleipold how about this?

I would "reciprocate" with vorburger/ch.vorburger.exec#275.

[fleipold]

Hi @vorburger,
Thank you for your PR (and the link back)
Two things:

1. Content-wise, for the link to be useful it would be nice to point out what use case your tool has as opposed to jproc this also applies to the link on your side. For jproc I tend to describe it as a library to run non-interactive external processes, that perform an operation on some (finite) input and produce some (finite) output. It could be described as "shelling-out".
2. Technically, the content of README.md is generated from this test case using docufier. mvn package should regenerate README.

So, the question is what is specific usecase for vorburger.exec? From having a very cursory look at its README it looks like its more suitable to launch and monitor long-running processes.