docs: update .all-contributorsrc [skip ci]

Author: allcontributors[bot]

.all-contributorsrc

@@ -2123,6 +2123,15 @@
  "contributions": [
  "code"
  ]
+ },
+ {
+ "login": "davidnixon",
+ "name": "David Nixon",
+ "avatar_url": "https://avatars.githubusercontent.com/u/7536103?v=4",
+ "profile": "https://github.com/davidnixon",
+ "contributions": [
+ "doc"
+ ]
  }
  ],
  "contributorsPerLine": 7,