docs: add mdboom as a contributor for maintenance (#615)

* 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

@@ -506,6 +506,15 @@
       "contributions": [
         "doc"
       ]
+    },
+    {
+      "login": "mdboom",
+      "name": "Michael Droettboom",
+      "avatar_url": "https://avatars.githubusercontent.com/u/38294?v=4",
+      "profile": "http://droettboom.com",
+      "contributions": [
+        "maintenance"
+      ]
     }
   ],
   "contributorsPerLine": 7,