Add launch instructions to README

[WalterGR]

I'm not seeing launch instructions in the README.

Per https://marketplace.visualstudio.com/items?itemName=ryzngard.carnation-colorpicker&ssr=false#qna :

Hey Kathleen,

You can launch by using View -> Other Windows -> Carnation.

The repo has this documentation, but unfortunately it's not on the extension page.

Thanks!