Document whether OS-independent/non-Linux Portal 2 bugs are welcome here

[strugee]

It's not very clear whether general bug reports are welcome here, as opposed to bugs that specifically have to do with the Linux port. The README kind of implies the latter, but the "not linux specific" label kind of implies the former? Maybe? This is super unclear. In general it's hard to know whether bugs should be reported here, in the Steam Forums, etc. (I originally looked in the Steam Forums since Steam Link bugs are supposed to go there apparently, but unlike the Steam Link forums the Portal 2 forums don't have a section specifically for bug reports.)