Update README.md

Author: ota-meshi

README.md

@@ -338,6 +338,7 @@ Please use GitHub's Issues/PRs.
 - `yarn cover` runs tests and measures coverage.
 - `yarn new [new-rule-name]` generate the files needed to implement the new rule.
 - `yarn update` runs in order to update readme and recommended configuration.
+- `yarn docs:watch` launch the document site in development mode.
 
 ### Test the Rule