Warn deprecation in README

[teamdandelion]

Rob Dodson told me this is deprecated. So the README should probably say that.

[otter-computer]

I'm pretty sure this will be officially deprecated when the carbon elements are released. Hopefully that shouldn't be long!

[neonsoftware]

Thanks for the Heads Up guys !

[otter-computer]

For a sneaky peek check out the Lab Elements Router 😉The carbon router was used in the app-layout demo and was recently changed to use the lab elements version instead so we can assume that this is what will soon be the carbon router.

[devinivy]

Thanks for the heads up, @danmane! And good snooping, @ThomasShaped. Also– yes, please addend the readme.

[addyosmani]

CC @tjsavage

[tjsavage]

Added a notice to the repo, thanks!