Update docs to recommend markdownlint-cli2 (#20)

* docs: update docs to recommend markdownlint-cli2

* Use markdownlint-cli2 in this library

* docs: instruct reloading window

Author: khiga8

.markdownlint-cli2.cjs

@@ -0,0 +1,12 @@
+const options = require('./index.js').init({
+    "default": false,
+    "heading-increment": true,
+    "no-alt-text": true,
+    "single-h1": true,
+    "no-emphasis-as-header": true,
+    "first-line-heading": true
+})
+module.exports = {
+    config: options,
+    customRules: ["./index.js"],
+}