Add documentation for generating a Visual Studio project on Windows

[hughbe]

Sorry, I think I accidently created a branch under apple/swift rather than hughbe/swift. Sorry

[compnerd]

@swift-ci please smoke test and merge

[compnerd]

@RUSshy swift already supports windows natively (without cygwin/mingw). This work is for building on windows rather than cross-compiling.

[hughbe]

@compnerd is correct. And the work for building on Windows is almost done - what remains is #5904 (when I fix the merge conflicts!)

There are obviously more fixes to be done that I am investigating. These include

• Fixing the unresolved symbol errors when linking swiftPrivate.dll
• Fixing the unresolved symbol errors when linking

class C {}

• Fixing an AccessViolationException acessing 0x7FFFFFFF... when running

print("Hello, World")

• Getting the tests building/working on Windows
• Trying to see if there is a portable solution for the REPL

Of course, @compnerd may have some ideas about these - I just don't want our work to keep duplicating!