Document practices around keeping empty directories in source control

[strugee]

Currently the Right Way™ to modify the filesystem in a generator is to use mem-fs-editor, and the Right Way™ to create a directory with mem-fs-editor is to directly use mkdirp. That works fine; however, the empty directory doesn't display in the list of files created (or conflicted, or whatever) at the end of the generator run. It'd be nice if I could tell Yeoman about these directories so it would display them in the list of created files.