use yarn: update README to include serve command and remove package-lock.json (#61)

Author: bercianor

README.md

@@ -24,6 +24,14 @@ $ yarn build
 
 This command generates static content into the `build` directory and can be served using any static contents hosting service.
 
+### Serve
+
+```
+$ yarn serve
+```
+
+This command serves the build generated with the previous command (`yarn build`).
+
 ### Deployment
 
 Using SSH: