Ignore build, MacOSX and Windows files

@ahmedbaracat rather than duplicating OS-specific rules for all projects, these live in https://github.com/github/gitignore/blob/master/Global/OSX.gitignore and https://github.com/github/gitignore/blob/master/Global/Windows.gitignore.

The build-related changes seem relevant here - any supporting docs around what these files represent and why they can be excluded from version control would be greatly appreciated!

@shiftkey Thanks for pointing that out.

The main reason for excluding build-related changes is not to hit the file size limit which, I believe, is 100 MB. Normally, games developed with unity, especially 3D ones, have large build files and you can always build the project when you fork/clone/download it :)

Hope this helps
APK: "the package file format used to distribute and install application software and middleware onto Google's Android operating system" https://en.wikipedia.org/wiki/Android_application_package

Unity package: https://help.market.envato.com/hc/en-us/articles/202500634-Building-a-UnityPackage

@ahmedbaracat those details are good enough for me - and the files are Unity-specific, not OS-specific as I first thought...