.gitignore all but the specified subdirectory

.gitignore

# Ignore all files except for a particular directory
*
# Don't ignore directories themselves, else we can't add anything to the repository
!*/
# Don't ignore any directory with the name "path"
!**/path/*
# or its subdirectories
!**/path/**/*

# And don't forget .gitignore itself
!.gitignore