You can include !-lines to whitelist files: a .gitignore with:
*
!included/
will exclude all, but the ‘included/’ directory
Note that if you want files matching a pattern to be un-ignored, in subdirectories, you will need to prevent the containing directories from getting ignored too. This should not pose a large problem, since git doesn’t actually track directories, only files (identified by a repository path).
Example:
*
!*/
!SOURCES
will ignore everything, except SOURCES in subdirectories.