scripts: document "updatedep.sh" script

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

scripts/updatedep.sh

@@ -7,5 +7,14 @@ if ! [[ "$0" =~ scripts/updatedep.sh ]]; then
 fi
 
 go get -v -u github.com/golang/dep/cmd/dep
-dep ensure -v
+
+if [[ -z "$1" ]]; then
+  echo "dep ensure on all packages"
+  dep ensure -v
+else
+  echo "dep update on" "$1"
+  # shellcheck disable=SC2086
+  dep ensure -v -update $1
+fi
+
 dep prune