Amend boot-setup docs to help with missing target
Amend boot-setup docs to help with missing target

Right after a fresh clone of `om-cookbook` repo the target directory
`om-cookbook/recipes/boot-setup/target` doesn't exist which makes `boot
dev` fail. Instuct the user to take an extra preparatory step that
establishes the directory in case it is missing.

Deraen/om-cookbook,omcljs/om-cookbook,argvader/om-cookbook,omcljs/om-cookbook,Deraen/om-cookbook,argvader/om-cookbook