fix contributing link (#21)

Author: dschaller

README.md

@@ -78,7 +78,7 @@ Thanks goes to these wonderful organizations! [[Contribute](https://opencollecti
 
 ## Contributors
 
-Thanks goes to these wonderful people! [[Become a contributor](CONTRIBUTING.md)].
+Thanks goes to these wonderful people! [[Become a contributor](https://github.com/react-hook-form/react-hook-form/blob/master/CONTRIBUTING.md)].
 
 <a href="https://github.com/react-hook-form/react-hook-form/graphs/contributors">
     <img src="https://opencollective.com/react-hook-form/contributors.svg?width=950" />