kpt pkg is intended to be used within a git repo - document it.
kpt pkg is intended to be used within a git repo - document it.

This change is an improvement to the documentation in order to address
https://github.com/GoogleContainerTools/kpt/issues/190.

Tested this by going through the new example flow starting from a
non-git directory.

GoogleContainerTools/kpt,GoogleContainerTools/kpt,GoogleContainerTools/kpt,GoogleContainerTools/kpt,GoogleContainerTools/kpt