docs/README: Add note to make OS requirement clear

[shymega-ct]

This PR adds a note to the README to make the OS requirement clearer.

It might be that people think WSL2 can run Buildstream/Buildbox, and this has not yet been officially tested.

[jjardon]

I think this add a line about windows not being officially supported but It's not very clear about what OSes are actually supported (my understanding is only Linux but I might be wrong)

Can we be more clear about that, please?

[shymega-ct]

I think this add a line about windows not being officially supported but It's not very clear about what OSes are actually supported (my understanding is only Linux but I might be wrong)

Can we be more clear about that, please?

I believe it's Linux-only, but WSL2 is an instance of Linux in a VM, so I think @juergbi's suggestion works well.

[cs-shadow]

I think this add a line about windows not being officially supported but It's not very clear about what OSes are actually supported ...

I concur, and would also suggest to phrase it in the positive direction of what's supported as opposed to enumerating what's not supported (which can be a never-ending list)

... (my understanding is only Linux but I might be wrong)
Can we be more clear about that, please?

I believe it's Linux-only, but WSL2 is an instance of Linux in a VM, so I think @juergbi's suggestion works well.

I believe Linux is the only thing that we test on and where BuildStream is used in earnest. In the past, there were weak attempts to make it work on MacOS and Windows to varying degrees but I don't think either of those work today in a meaningful way.

Other than WSL, Docker Desktop (or Podman Desktop and such) can be a viable option in a similar "may be used but officially unsupported" category. The contrib/bst-here script was created to that end and can be potentially referenced here.