Adding my name to contributors list

Author: mattatmet

CONTRIBUTORS.md

@@ -38,4 +38,5 @@
 | mo-lucy-gordon      | Lucy Gordon         | Met Office                       | 2026-03-11 |
 | iboutle             | Ian Boutle          | Met Office                       | 2026-03-04 |
 | Pierre-siddall      | Pierre Siddall      | Met Office                       | 2026-03-02 |
-| jirudaya            | Jaffery Irudayasamy | University of Edinburgh          | 2026-03-11 |
+| jirudaya            | Jaffery Irudayasamy | University of Edinburgh          | 2026-03-11 |
+| mattatmet           | Matthew Walker      | Met Office                       | 2026-04-21 |