functional: add stub package documentation
functional: add stub package documentation

Fixes Travis builds using Go 1.5 which attempts to `go get ./...` but
this causes a fatal error if a package directory is found without any
buildable code in it.

bviswana101/fleet,dalbani/fleet,paveltiunov/fleet,dongsupark/fleet,jonboulle/fleet,coreos/fleet,dongsupark/fleet,endocode/fleet,mookjp/fleet,improbable-io/fleet,bviswana101/fleet,giantswarm/fleet,improbable-io/fleet,paveltiunov/fleet,giantswarm/fleet,endocode/fleet,dalbani/fleet,coreos/fleet,giantswarm/fleet,esatterwhite/fleet,mookjp/fleet,endocode/fleet,jonboulle/fleet,dongsupark/fleet,improbable-io/fleet,jonboulle/fleet,dalbani/fleet,coreos/fleet,esatterwhite/fleet