Update README.md (#1553)
Update README.md (#1553)

Make some minor modifications to make contributing clearer.
CupOfTea696/prism,CupOfTea696/prism,CupOfTea696/prism,PrismJS/prism,CupOfTea696/prism,PrismJS/prism,PrismJS/prism,CupOfTea696/prism,PrismJS/prism,PrismJS/prism