docs: ✏️ update readme

Author: ttskch

README.md

@@ -12,17 +12,30 @@
 
 ## Installation
 
+### CDN
+
+```html
+<link rel="stylesheet" href="/path/to/select2.css">
+<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/@ttskch/select2-bootstrap4-theme/dist/select2-bootstrap4.css">
+```
+
+### Manually
+
 ```bash
+# npm
 $ npm install @ttskch/select2-bootstrap4-theme
-```
 
-## Usage
+# yarn
+$ yarn add @ttskch/select2-bootstrap4-theme
+```
 
 ```html
 <link rel="stylesheet" href="/path/to/select2.css">
 <link rel="stylesheet" href="/path/to/select2-bootstrap4.css">
 ```
 
+## Usage
+
 ```js
 $('select').select2({
     theme: 'bootstrap4',