Mention OS support in docs

[ulf5]

Hello,

First I'd like to say I've only been using mmids for a very short time, but so far it's been working great!

Anyway, I couldn't find any mention of which OS are supported in the docs and all the paths in the examples are Windows style so I was a bit worried it wouldn't work on Linux. But after building it from source it seems to work just fine. I think it would be good to mention which OS are supported somewhere in the docs.

[KallDrexx]

So glad it's working well for you!

Good call on the OS documentation. While I developed it in windows I tried to make sure it was hopefully as portable as possible. I probably should have tested it in WSL (didn't think about it). I'll add a bit to the front page to mention cross platform ness.