Compilation instructions for Windows in the README

[taywater]

With the Windows build process being explicitly supported (#101), adding compilation instructions for Windows in the README would be tremendously helpful.

[aintitfun]

+1, on msys/mingw the resulting dll works fine but on visual studio I have some dll dependency errors with the resulting dll.

[mlt]

@aintitfun You need to match VC runtime to that used by EDB binary. That is you might have problems using newer Visual Studio for older PostgreSQL and vice versa. Also make sure you use Release build if you want to use it on another machine.
It is really a single line. You can try opening x64 Native Tools Command Prompt for VS and use

msbuild /p:PlatformToolset=v141 /p:configuration=Release /p:platform=x64 odbc_fdw.vcxproj

This should work for PG 12.