Add MD041 with tests.

lib/rules.js

@@ -779,5 +779,28 @@ module.exports = [
         }
       });
     }
+  },
+
+  {
+    "name": "MD041",
+    "desc": "First line in file should be a top level header",
+    "tags": [ "headers" ],
+    "func": function MD041(params, errors) {
+      var firstHeader = null;
+      params.tokens.every(function forToken(token) {
+        if (token.type === "heading_open") {
+          firstHeader = token;
+          return false;
+        } else if (token.lineNumber > 1) {
+          return false;
+        }
+        return true;
+      });
+      if (!firstHeader ||
+          (firstHeader.lineNumber !== 1) ||
+          (firstHeader.tag !== "h1")) {
+        errors.push(1);
+      }
+    }
   }
 ];