Update README.md

Adding a link to the skillsets builder docs.

Author: D1M1TR10S

README.md

@@ -102,5 +102,6 @@ This bot provides a passthrough to a couple of other APIs:
 
 ## Documentation
 - [Using Copilot Extensions](https://docs.github.com/en/copilot/using-github-copilot/using-extensions-to-integrate-external-tools-with-copilot-chat)
+- [About skillsets](https://docs.github.com/en/copilot/building-copilot-extensions/building-a-copilot-skillset-for-your-copilot-extension/about-copilot-skillsets)
 - [About building Copilot Extensions](https://docs.github.com/en/copilot/building-copilot-extensions/about-building-copilot-extensions)
 - [Set up process](https://docs.github.com/en/copilot/building-copilot-extensions/setting-up-copilot-extensions)