Document manual text-icu building on Windows

[nponeccop]

See #599 - basically text-icu dependency fails out of the box because of missing C dependency.

So some build instruction would be nice.

[alanz]

@nponeccop, can we just add your instructions:

• go to %USERPROFILE%\AppData\Local\Programs\stack\x86_64-windows\msys2-20150512\
• run msys2_shell.bat
• in the shell run pacman -Sy mingw-w64-x86_64-icu
• then it should run out of the box (i.e. without passing pathes anywhere)

If you run into troubles, e.g. if you use 32-bit GHC, you can do pacman -Ss icu to see the various icu-related packages available

?

[nponeccop]

LGTM