docs: clear ambiguity about CDN usage

Add unpkg as an alternative cdn and clear ambiguity about CDN usage
It allows to skip version tag (it uses a latest version as default) and it's appealing due to its conciseness.

Closes  #742

README.md

@@ -69,7 +69,11 @@ You can also use one of several CDNs available:
 
         https://cdnjs.cloudflare.com/ajax/libs/showdown/<version tag>/showdown.min.js
 
+* unpkg
+
+        https://unpkg.com/showdown/dist/showdown.min.js
 
+*Note*: replace `<version tag>` with an actual full length version you're interested in e.g. `1.9.0`
 ## Browser Compatibility
 
 Showdown has been tested successfully with: