docs: add notice about CDN for v1

Author: heenakwag

docs/v2.0-migration-guide.md

@@ -2,12 +2,14 @@
 
 ## Install
 
-The v2.0 should be installed using the `npm` command. CodeSnippet does not support bower and CDN anymore.
+The v2.0 should be installed using the `npm` command.
 
 ``` sh
 $ npm install --save tui-code-snippet
 ```
 
+CodeSnippet does not support bower and CDN anymore. However, CDN for v1.x still has been provided. It should be noted that **the latest version of CDN is the last release of v1**, not the latest version.
+
 ## Usage
 
 The v2.0 supports importing a single function, so you can import only functions that you need.
@@ -108,4 +110,4 @@ From v2.0, the below functions are not provided anymore.
 * hasEncodableString
 * getDuplicatedChar
 * timestamp
-* Popup
+* Popup