Use with a Psalm baseline gives misleading results, with an inflated MSI score

[bdsl]

I started using infection on a project via this plugin, but eventually after doing some investigations to learn exactly how Psalm was killing some mutants I found that mutants are shown as killed by Psalm errors even if those errors are in the project's Psalm baseline file, which means that Psalm does not report any error when it is run after the mutation is applied.

I don't know if it's feasible to make this plugin work with a baseline, but it is feasible to try to prevent usage with a baseline, by adding a warning about this issue to the readme and/or by making the plugin detect the presence of a baseline and refuse to run.

I'd be happy to make a PR to implement one or both of those changes if maintainers here think that would be helpful.

[Ocramius]

IMO I'd start with a small functional test containing an obvious baseline entry: from there, we could discuss approaches 🤔