Group update description (#859)

* Added groupUpdateDescription

* Updated README

Co-authored-by: Ilya Borodin <ilya.borodin@botconversa.com.br>

README.md

@@ -542,6 +542,10 @@ Of course, replace ``` xyz ``` with an actual ID.
     ``` ts
     await conn.groupUpdateSubject("abcd-xyz@g.us", "New Subject!")
     ```
+- To change the group's description
+    ``` ts
+    await conn.groupUpdateDescription("abcd-xyz@g.us", "New Description!")
+    ```
 - To change group settings
     ``` ts
     // only allow admins to send messages