Updated Ubuntu 18 Installation Instruction

[frank-qcd-qk]

Per observation, some minor tweaks based upon current version:

• Added the verification step of which dts so that user can verify their installation
• Re-ordered the warning regarding not to do SUDO for pip install to further highlight the issue.
• Added reference to official docker installation instruction per Duckiebot OPmanual Docker section. Link: https://docs.duckietown.org/DT19/opmanual_duckiebot/out/laptop_setup.html