Removes readme and contributing.md from generate heading lint (#4295)

* Removes readme and contributing.md from generate heading

* lint only markdowns in source

beta/package.json

@@ -100,6 +100,6 @@
   },
   "lint-staged": {
     "*.{js,ts,jsx,tsx,css}": "yarn prettier",
-    "*.md": "yarn fix-headings"
+    "src/**/*.md": "yarn fix-headings"
   }
 }