cmd/vendor/README: temporary instruction for update
Until vgo sorts out and cleans up the vendoring process.
Ran govendor to update packages the cmd/pprof depends on
which resulted in deletion of some of unnecessary files.
Change-Id: Idfba53e94414e90a5e280222750a6df77e979a16
Reviewed-on: https://go-review.googlesource.com/114079
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com> Reviewed-by: Daniel Theophanes <kardianos@gmail.com>