Better install instructions for Windows users

[fingolfin]

1. say more precisely which Julia binary to download ("look for Linux 64bit glibc"), or even better, provide a direct download link
2. say how to actually download the result inside WSL
3. say how/where to extract it so that the user can access it from anywhere ("add it to PATH")
4. alternatively to the above: suggest a "one click install method" if there is one (probably not...) or at least one that is simpler; e.g. it might be easier to tell people something like sudo apt-get install python3 && pip3 install jill --user -U && jill install or so (to be tested!!!!)
5. It would be great to have a way to add an icon to the desktop or start menu which users can select to start a Julia session (inside WSL), or even an Oscar session

[fingolfin]

I guess this partially duplicates issue #150 but I don't have time to properly join them right now