Installation notes to build Unison on Windows systems

We provide two options for building Unison on MS Windows.  Both
options require the Cygwin layer to be able to use a few GNU tools as
well as the OCaml distribution version.  The options differ in the C
compiler employed: MS Visual C++ (MSVC) vs Cygwin GNU C.

Tradeoff?
. Only the MSVC option can produce statically linked Unison executable.
. The Cygwin GNU C option requires only free software.

The files "INSTALL.win32-msvc" and "INSTALL.win32-cygwin-gnuc" describe
the building procedures for the respective options.
