Add "Things made with periodic-table" section to README

[andrejewski]

I want to see what has been made with this if anyone is willing to share.

[a-ogilvie]

I made a chrome extension! I made it for my final project as part of Harvard's CS50x programme.
https://chrome.google.com/webstore/detail/chemical-information/fddoccbkbikfomalnifggoajpghchjcp