Remove 'AUTHORS.txt' from README.md

Relates to pion/.goassets#185

README.md

@@ -31,7 +31,7 @@ We are always looking to support **your projects**. Please reach out if you have
 If you need commercial support or don't want to use public methods you can contact us at [team@pion.ly](mailto:team@pion.ly)
 
 ### Contributing
-Check out the [contributing wiki](https://github.com/pion/webrtc/wiki/Contributing) to join the group of amazing people making this project possible: [AUTHORS.txt](./AUTHORS.txt)
+Check out the [contributing wiki](https://github.com/pion/webrtc/wiki/Contributing) to join the group of amazing people making this project possible
 
 ### License
 MIT License - see [LICENSE](LICENSE) for full text