docs(README.md): point to CONTRIBUTING.md at root of directory (#66)

## Summary

This updates the README's reference to the CONTRIBUTING.md file to the
correct location at root, rather than its previous location in
`.github/CONTRIBUTING.md`.

Author: francisfuzz

README.md

@@ -461,7 +461,7 @@ While implementing the lower-level functionality, we also dream big: what would
 
 ## Contributing
 
-Please see [CONTRIBUTING.md](.github/CONTRIBUTING.md)
+Please see [CONTRIBUTING.md](CONTRIBUTING.md)
 
 ## License