Markdown Linting - Improving rules

Author: swisskyrepo

.github/.markdownlint.json

@@ -1,5 +1,9 @@
 {
+    "default": true,
     "MD013": false,
+    "no-duplicate-heading": {
+      "siblings_only": true
+    },
     "ul-indent": {
       "indent": 4
     }