feat: custom lint rule, fixed readme

.markdownlint-cli2.jsonc

@@ -0,0 +1,14 @@
+{
+  "customRules": [".markdownlint-rules/one-sentence-per-line.mjs"],
+  "config": {
+    "default": true,
+    "heading-style": { "style": "atx" },
+    "ul-indent": { "indent": 2 },
+    "line-length": false,
+    "no-duplicate-heading": { "siblings_only": true },
+    "no-inline-html": {
+      "allowed_elements": ["br", "code", "details", "summary", "img", "picture", "source"]
+    },
+    "first-line-heading": true
+  }
+}