docs: Add a step in CONTRIBUTING.md docs (#722)

* Add missing lerna step in development workflow

* Update the command for building the content of the library

CONTRIBUTING.md

@@ -21,11 +21,10 @@ Purgecss use [SemVer](http://semver.org/) for versioning.
 
 ### Development Workflow
 
-After cloning Purgecss, run `npm i` to fetch its dependencies. Then, you can run
+After cloning Purgecss, run `npm i && npm run bootstrap` to fetch its dependencies. Then, you can run
 several commands:
 
-* `npm run dev` will build cjs and es module of all PurgeCSS packages in their `lib` folder and
-  watch for changes.
+* `npm run build` will build cjs and es module of all PurgeCSS packages in their `lib` folder.
 * `npm run lint` checks the code style.
 * `npm test` runs the complete test suite.
 * `npm test -- --watch` runs an interactive test watcher.