docs(README): add links to official Copilot Extensions documentation (copilot-extensions#108)

Author: D1M1TR10S

README.md

@@ -487,6 +487,14 @@ if (functionCall) {
 }
 ```
 
+## Copilot Extensions Documentation
+
+- [Using Copilot Extensions](https://docs.github.com/en/copilot/using-github-copilot/using-extensions-to-integrate-external-tools-with-copilot-chat)
+- [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)
+- [Communicating with the Copilot platform](https://docs.github.com/en/copilot/building-copilot-extensions/building-a-copilot-agent-for-your-copilot-extension/configuring-your-copilot-agent-to-communicate-with-the-copilot-platform)
+- [Communicating with GitHub](https://docs.github.com/en/copilot/building-copilot-extensions/building-a-copilot-agent-for-your-copilot-extension/configuring-your-copilot-agent-to-communicate-with-github)
+
 ## Dreamcode
 
 While implementing the lower-level functionality, we also dream big: what would our dream SDK for Coplitot extensions look like? Please have a look and share your thoughts and ideas: