Add instructions on howto easily use it on Windows

[gussen]

Add instructions on howto easily use it on Windows

The easy way to build on Windows is to install libbotan package using MSYS2/MinGW and add some information to project.cabal.
MSYS2/MinGW64 is installed by GHCUP/GHC on Windows.

Here are the instructions (Paths might be different on your machine)

Install libbotan and dependencies in MSYS2. Start a MSYS2 shell by executing C:\ghcup\msys64\msys2.exe
or C:\ghcup\msys64\msys2_shell.cmd -msys

# If needed update the package database and ugrade all installed packages
pacman -Syu
pacman -Su

# Install libbotan and it's dependencies
pacman -S mingw-w64-x86_64-libbotan

Add the botan-low dependency to the projects cabal file

build-depends:   base  ^>=4.17.2.1
               , botan-low

Add this to cabal.project so that the needed parts of the Botan C++ library can
be found when building.

packages: .

package botan-bindings
extra-include-dirs: C:\ghcup\msys64\mingw64\include\botan-3
extra-lib-dirs: C:\ghcup\msys64\mingw64\bin, C:\ghcup\msys64\mingw64\lib

Build and run from PowerShell

 # Add libbotan-3.dll location to Path for this session
 # so that the executable can find the dll.
 $env:Path += ";C:\ghcup\msys64\mingw64\bin"
 cabal build
 cabal run