Added @steve-gregory for nullable slug relation tests.

See: #585

docs/topics/credits.md

@@ -91,6 +91,7 @@ The following people have helped make REST framework great.
 * Richard Wackerbarth - [wackerbarth]
 * Johannes Spielmann - [shezi]
 * James Cleveland - [radiosilence]
+* Steve Gregory - [steve-gregory]
 
 Many thanks to everyone who's contributed to the project.
 
@@ -217,3 +218,4 @@ You can also contact [@_tomchristie][twitter] directly on twitter.
 [wackerbarth]: https://github.com/wackerbarth
 [shezi]: https://github.com/shezi
 [radiosilence]: https://github.com/radiosilence
+[steve-gregory]: https://github.com/steve-gregory