Explain yarn link in README

Author: phortx

README.md

@@ -94,6 +94,10 @@ You can see query examples in the [project wiki](https://github.com/vuex-orm/vue
 
 ## Contribution
 
+To test this plugin in your existing project, you can use `yarn link` functionality. Run `yarn link` in your local
+vuex-orm-apollo directory and run `yarn link @vuex-orm/plugin-apollo` in your project dir.
+
+
 ```bash
 $ yarn run build
 ```