Improve linking on Windows

As noted on Discord, I ended up having to do this as I couldn't get my hands on a pre-built import library that worked for me.

I now suspect the issue was that I was trying to build against a 32-bit import library when building a 64-bit binary, but there aren't any 64-bit pre-built packages on videolan.org/vlc which include an import library.

One benefit of doing things the way I've done them here is that it should now be possible to point vlc-rs at any regular (compatible) VLC install, and it'll link properly.

I've also added some rough build instructions to the README, but I'm not particularly knowledgeable about how stuff is expected to be done with VLC, so I could be getting some things wrong there.