Update readme

README.md

@@ -2,7 +2,15 @@
 
 ## How to use Cookie Extender tag
 
-TBD
+Use Cookie Extender tag template to extend cookies lifetime using server Google Tag Manager container. To set up a tag, you need to add:
+
+- Cookie Name
+- Lifetime in seconds
+- Create backup cookies and restore them in case main cookies are not found - this option allows tag to create backup cookies and restore them. For example, for _ga cookie, _ga_backup will be created. When _ga cookie expires, Cookie Extender tag will restore _ga cookie using _ga_backup.
+
+## How to use Cookie Extender tag
+
+- [Extend cookie lifetime using server-side Google Tag Manager](https://stape.io/blog/extend-cookie-lifetime-using-server-side-google-tag-manager)
 
 ## Open Source