chore(docs): add svbutko as a contributor (#443)

Adds @svbutko as a contributor for doc.

This was requested by Titozzz [in this comment](https://github.com/react-native-community/react-native-webview/pull/442#issuecomment-475252579)

.all-contributorsrc

         "code",
         "doc"
       ]
+    },
+    {
+      "login": "svbutko",
+      "name": "Sergei Butko",
+      "avatar_url": "https://avatars2.githubusercontent.com/u/14828004?v=4",
+      "profile": "https://www.linkedin.com/in/svbutko/",
+      "contributions": [
+        "doc"
+      ]
     }
   ],
   "contributorsPerLine": 7