docs: add pre-commit-ci as a contributor for maintenance (#480)

Adds @pre-commit-ci as a contributor for maintenance.

This was requested by samcunliffe [in this
comment](#479 (comment))

---------

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

.all-contributorsrc

@@ -249,6 +249,15 @@
       "contributions": [
         "bug"
       ]
+    },
+    {
+      "login": "pre-commit-ci",
+      "name": "pre-commit.ci",
+      "avatar_url": "https://avatars.githubusercontent.com/u/64617429?v=4",
+      "profile": "https://pre-commit.ci",
+      "contributions": [
+        "maintenance"
+      ]
     }
   ]
 }

README.md

@@ -59,6 +59,7 @@ Python packages with our recommended tooling set up and ready to go.
       <td align="center" valign="top" width="14.28%"><a href="https://github.com/robertvi"><img src="https://avatars.githubusercontent.com/u/456100?v=4?s=100" width="100px;" alt="Robert Vickerstaff"/><br /><sub><b>Robert Vickerstaff</b></sub></a><br /><a href="#doc-robertvi" title="Documentation">📖</a></td>
       <td align="center" valign="top" width="14.28%"><a href="http://dpshelio.github.io"><img src="https://avatars.githubusercontent.com/u/963242?v=4?s=100" width="100px;" alt="David Pérez-Suárez"/><br /><sub><b>David Pérez-Suárez</b></sub></a><br /><a href="#code-dpshelio" title="Code">💻</a> <a href="#question-dpshelio" title="Answering Questions">💬</a></td>
       <td align="center" valign="top" width="14.28%"><a href="https://github.com/LLapira"><img src="https://avatars.githubusercontent.com/u/48060852?v=4?s=100" width="100px;" alt="llapira"/><br /><sub><b>llapira</b></sub></a><br /><a href="#bug-llapira" title="Bug reports">🐛</a></td>
+      <td align="center" valign="top" width="14.28%"><a href="https://pre-commit.ci"><img src="https://avatars.githubusercontent.com/u/64617429?v=4?s=100" width="100px;" alt="pre-commit.ci"/><br /><sub><b>pre-commit.ci</b></sub></a><br /><a href="#maintenance-pre-commit-ci" title="Maintenance">🚧</a></td>
     </tr>
   </tbody>
 </table>