Helpful install documentation needed

[geokala]

Install documentation is needed which covers supported OSes.

[geokala]

This should point users to Musescore if they want to show score (note: musescore 3 is the major version currently supported).

Windows users don't need to do anything for midi playback (probably).

It should point Mac users towards VLC for midi playback.

It should point Linux users towards timidity for midi playback.

[djaychela]

I've spent a bit of time this morning updating the README and done a PR for what I've come up with - hope that's OK (I'm pretty green at all this side of things!)