update MAINTENANCE.md

Author: gabrielburnworth

MAINTENANCE.md

@@ -9,7 +9,10 @@ git merge v<new upstream release>
 git push origin main
 cd ..
 git clone [email protected]:Nerves-Project/nerves_system_br -b v<latest release>
-make system # this will take a while.
+./nerves_system_br/create-build.sh ./farmbot_system_rpi3/nerves_defconfig BUILD_RPI3
+cd BUILD_RPI3
+make system # this will take a while. dependencies may need to be installed/upgraded
+cd ..
 # create github release.
 # upload system artifact.
 cd farmbot_system_rpi3