Document HASHLINK variable used by VS templates

The Hashlink haxelib library can be used to generate Visual Studio project files. These project files make use of a `HASHLINK` environment variable ([see here](https://github.com/HaxeFoundation/hashlink/blob/a46d1483cdfe7e6356dd0ffa3de732853f1a43d1/other/haxelib/templates/vs2019/__file__.vcxproj#L74-L90)), pointing to the location where Hashlink is installed.

It would be good for this environment variable to be documented somewhere so that people are able to compile the project files that are generated. Furthermore, later down the road it would be good for this variable to be set automatically when installing Hashlink on Windows.