Overhaul the README.md

Author: ralish

.markdownlint.json

@@ -3,5 +3,7 @@
     "heading-style": { "style": "setext_with_atx" },
     "no-trailing-spaces": { "br_spaces": 2 },
     "line-length": false,
-    "no-trailing-punctuation": { "punctuation": ".,;:!" }
+    "no-duplicate-heading": { "siblings_only": true },
+    "no-trailing-punctuation": { "punctuation": ".,;:!" },
+    "no-inline-html": { "allowed_elements": ["sup"] }
 }