Update README to reflect release of WebGPU on chrome

[RyanMcEntire]

I'm writing to remind you of what you wrote in the readme, that you would update once it was released.

WebGPU is now available in chrome and chrome for android, and ships with the full release, enabled by default except for Linux, which requires a flag to enable vulkan.

https://github.com/gpuweb/gpuweb/wiki/Implementation-Status

Thanks for the great videos. The shader one was very helpful.