docs: add dbismut as a contributor (testing-library#259)

* docs: update README.md

* docs: update .all-contributorsrc

Author: allcontributors[bot]

.all-contributorsrc

@@ -546,6 +546,15 @@
       "contributions": [
         "code"
       ]
+    },
+    {
+      "login": "dbismut",
+      "name": "David",
+      "avatar_url": "https://avatars2.githubusercontent.com/u/5003380?v=4",
+      "profile": "https://github.com/dbismut",
+      "contributions": [
+        "code"
+      ]
     }
   ],
   "repoHost": "https://github.com"