Add a note about how cached directories work

[sanderson]

Add a note about exactly how cached directories work, more specifically something like “Cached directories should only be directories which are generated by the application, not directories that are in your code base. That is because any directories which match will not be deployed from your code base and as such will be missing from the deployed application.”

[danhunsaker]

"will not be deployed from your code base [on your host system, but rather from the cache generated inside the development container,] and as such [anything outside that cache] will be missing"