docs: add marcelbarner as a contributor (#342)

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

Author: allcontributors[bot]

.all-contributorsrc

@@ -591,6 +591,16 @@
       "contributions": [
         "doc"
       ]
+    },
+    {
+      "login": "marcelbarner",
+      "name": "Marcel Barner",
+      "avatar_url": "https://avatars.githubusercontent.com/u/12788744?v=4",
+      "profile": "https://github.com/marcelbarner",
+      "contributions": [
+        "code",
+        "test"
+      ]
     }
   ],
   "repoHost": "https://github.com",

README.md

@@ -1186,6 +1186,7 @@ Thanks goes to these people ([emoji key][emojis]):
     <td align="center"><a href="https://nickmccurdy.com/"><img src="https://avatars0.githubusercontent.com/u/927220?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Nick McCurdy</b></sub></a><br /><a href="https://github.com/testing-library/jest-dom/commits?author=nickmccurdy" title="Documentation">📖</a> <a href="https://github.com/testing-library/jest-dom/issues?q=author%3Anickmccurdy" title="Bug reports">🐛</a> <a href="https://github.com/testing-library/jest-dom/commits?author=nickmccurdy" title="Code">💻</a></td>
     <td align="center"><a href="https://obedparla.com/"><img src="https://avatars1.githubusercontent.com/u/10674462?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Obed Marquez Parlapiano</b></sub></a><br /><a href="https://github.com/testing-library/jest-dom/commits?author=obedparla" title="Documentation">📖</a></td>
     <td align="center"><a href="https://github.com/calebeby"><img src="https://avatars.githubusercontent.com/u/13206945?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Caleb Eby</b></sub></a><br /><a href="https://github.com/testing-library/jest-dom/commits?author=calebeby" title="Documentation">📖</a></td>
+    <td align="center"><a href="https://github.com/marcelbarner"><img src="https://avatars.githubusercontent.com/u/12788744?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Marcel Barner</b></sub></a><br /><a href="https://github.com/testing-library/jest-dom/commits?author=marcelbarner" title="Code">💻</a> <a href="https://github.com/testing-library/jest-dom/commits?author=marcelbarner" title="Tests">⚠️</a></td>
   </tr>
 </table>