gx import: add example to CLI help.
gx import: add example to CLI help.

The first time I used import I had to lookup the gx readme to
figure out how to actully use it. It was not clear if it expects:
  * mHash
  * mHash/pkgname
  * ipfs/mHash
  * fs:/ipfs/mHash
  * etc.

whyrusleeping/gx,whyrusleeping/gx