docs: add tacaswell as a contributor for maintenance (#3273)

* docs: update README.md [skip ci]

* docs: update .all-contributorsrc [skip ci]

---------

Co-authored-by: allcontributors[bot] <46447321+allcontributors[bot]@users.noreply.github.com>

.all-contributorsrc

@@ -483,6 +483,15 @@
  "contributions": [
  "code"
  ]
+ },
+ {
+ "login": "tacaswell",
+ "name": "Thomas A Caswell",
+ "avatar_url": "https://avatars.githubusercontent.com/u/199813?v=4",
+ "profile": "https://tacaswell.github.io",
+ "contributions": [
+ "maintenance"
+ ]
  }
  ],
  "contributorsPerLine": 7,

README.md

@@ -230,6 +230,7 @@ Thanks especially to the gracious help of Awkward Array contributors (including
   <td align="center" valign="top" width="14.28%"><a href="https://github.com/pfackeldey"><img src="https://avatars.githubusercontent.com/u/18463582?v=4?s=100" width="100px;" alt="Peter Fackeldey"/><br /><sub><b>Peter Fackeldey</b></sub></a><br /><a href="https://github.com/scikit-hep/awkward/commits?author=pfackeldey" title="Code">💻</a></td>
   <td align="center" valign="top" width="14.28%"><a href="http://www.ariostas.com"><img src="https://avatars.githubusercontent.com/u/7596837?v=4?s=100" width="100px;" alt="Andres Rios Tascon"/><br /><sub><b>Andres Rios Tascon</b></sub></a><br /><a href="https://github.com/scikit-hep/awkward/commits?author=ariostas" title="Code">💻</a></td>
   <td align="center" valign="top" width="14.28%"><a href="https://github.com/maxymnaumchyk"><img src="https://avatars.githubusercontent.com/u/70752300?v=4?s=100" width="100px;" alt="maxymnaumchyk"/><br /><sub><b>maxymnaumchyk</b></sub></a><br /><a href="https://github.com/scikit-hep/awkward/commits?author=maxymnaumchyk" title="Code">💻</a></td>
+  <td align="center" valign="top" width="14.28%"><a href="https://tacaswell.github.io"><img src="https://avatars.githubusercontent.com/u/199813?v=4?s=100" width="100px;" alt="Thomas A Caswell"/><br /><sub><b>Thomas A Caswell</b></sub></a><br /><a href="#maintenance-tacaswell" title="Maintenance">🚧</a></td>
  </tr>
  </tbody>
 </table>