Add windows tips in documentation

[moltenform]

This workaround resolved the file-not-found errors I got otherwise.

(What's happening is that when there are parameters beginning with a leading slash like /Bicubic , msys mistakenly thinks it is a filepath and adds a prefix. The exported flags turn off that behavior (for git-for-windows and msys2 at least, vanilla msys1 still might run into the problem).)