Add purgecss to remove unused css, purgecss config and build config

README.md

@@ -9,10 +9,10 @@ yarn install
 
 Then you can run the development mode with hot reload by:
 ```sh
-gatsby develop
+yarn run develop
 ```
 
 If you want to make a build, do instead:
 ```sh
-gatsby build
-```
+yarn run build
+```