STYLE: Add more detail to the vxl update instructions

State explicitly that the upstream extraction script should be run in an
outside tree and the results moved into the temporary directory for
commit of a new upstream snapshot.

Change-Id: I1a9742a9154c8d45bb7bb530cd16e42687268a43
Merge topic 'vxl-update-instructions'

e245f0b STYLE: Add more detail to the vxl update instructions