add myself to CONTRIBUTORS.md

Author: nwh

CONTRIBUTORS.md

@@ -7,5 +7,6 @@ Many thanks to the following developers for contributing to this project:
 - [Geoff Jukes](https://github.com/geoffjukes)
 - [Giampaolo](https://github.com/gpcimino)
 - [Martin Larralde](https://github.com/althonos)
+- [Nick Henderson](https://github.com/nwh)
 - [Will McGugan](https://github.com/willmcgugan)
 - [Zmej Serow](https://github.com/zmej-serow)