update contributing link
update contributing link

file:a/README.md -> file:b/README.md
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@
 ## Contributing
 Changes and improvements are more than welcome! Feel free to fork and open a pull request. Please make your changes in a specifically made branch and request to pull on `master`! If you can, please make sure the game fully works before sending the PR, as that will help speed up the process.
-You can find the same information in the [contributing guide.](https://github.com/gabrielecirulli/2048/blob/master/CONTRIBUTING.txt)
+You can find the same information in the [contributing guide.](https://github.com/gabrielecirulli/2048/blob/master/CONTRIBUTING.md)
 ## License
 2048 is licensed under the [MIT license.](https://github.com/gabrielecirulli/2048/blob/master/LICENSE.txt)