Add note on supported OS platforms to README

As libgsf is bundled with gsfpy as a shared object (*.so) library, it will only run on Linux platforms. Add a note on this to the README.

[kjerram]

This would be a very helpful note for non-experts like myself. I would love to use gsfpy on a Windows machine and it took me, admittedly, quite some time to realize why I couldn't.

Is it possible to run gsfpy with Windows through some other workaround?

[anthonyklemm]

This would be a very helpful note for non-experts like myself. I would love to use gsfpy on a Windows machine and it took me, admittedly, quite some time to realize why I couldn't.

Is it possible to run gsfpy with Windows through some other workaround?

I'd also love to know if anyone has figured out how to use gsfpy in a windows env, or at least knows of any other python libraries that can read gsf.

[wrenoud]

@anthonyklemm I likely can help. I will email you.

And for future readers, if I figure anything out I'll make sure to share here.