Added link to Rico's tweets

Author: mjpost

README.md

@@ -635,7 +635,7 @@ SacreBLEU is licensed under the [Apache 2.0 License](LICENSE.txt).
 
 # Credits
 
-This was all Rico Sennrich's idea.
+This was all [Rico Sennrich's idea](https://twitter.com/RicoSennrich/status/883246242763026433)
 Originally written by Matt Post.
 New features and ongoing support provided by Martin Popel (@martinpopel) and Ozan Caglayan (@ozancaglayan).