Update the sysroot subtree pull command in README
Update the sysroot subtree pull command in README

* Changes from the upstream repo should not show up as individual
  commits, but instead as a single squashed commit

* The pull command should not perform a commit automatically

cs-education/sysassets,cs-education/sysassets,cs-education/sysassets,cs-education/sysassets,cs-education/sysassets,cs-education/sysassets,cs-education/sysassets