Create proper build instructions for Windows

[saxbophone]

*nix-likes are pretty straightforward with CMake, but I need to properly suss out how to do it on Windows, and check that it works.

I'm only going to be able to test it on Windows 10, so I hope my findings here will be transferable for older versions of the OS.

[saxbophone]

Got a successful build of the library on a Windows 10 VM. Tests also passing. Will write up instructions later.

[saxbophone]

Ok, it turns out I was building it as 32-bit with mingw-32 :( This won't work as libsaxbospiral is 64-bit only.

If anyone with more experience building 64-bit CMake projects on windows wants to try this out, invitation is open.

[saxbophone]

Blocked until #138 is released as making the library support 32-bit arch's will simplify this greatly (tricky to get the 64-bit MinGW toolchain on Windows).

[saxbophone]

No longer blocked as #138 is now released