[UI] Add instruction to install WebUI and simulator

[tbkoay]

It would be useful for folks can play with the WebUI before providing you with more relevant feedback.

Please provide

• [x] Include reference to the technologies used for newbies
• [x] Simple installation guide
• [x] Where to get the simulator and installation process @ngyewch

[ngyewch]

I've updated the README in https://github.com/org-arl/r2c2/pull/5

The simulator can be downloaded from: https://github.com/org-arl/r2c2/releases/download/starfish-simulator-20200417-1230/starfish-simulator.zip