docs(README): now power-assert became a tool, not library
docs(README): now power-assert became a tool, not library

link to latest talk slide.
you don't need `require('power-assert')` any more.

power-assert-js/power-assert,twada/power-assert,power-assert-js/power-assert,falsandtru/power-assert,twada/power-assert,falsandtru/power-assert