Update publish section of README.md
Update publish section of README.md

`yarn publish` is already supported prepack script in yarn 1.0.0.
(and prepublish scripts is running before packing in yarn 1.2.0)

See:
https://github.com/yarnpkg/yarn/pull/4193
https://github.com/yarnpkg/yarn/pull/4553

yhatt/markdown-it-incremental-dom