Added method to join group (#417)

* Added method to join group

* Added method to join group in README

README.md

@@ -527,6 +527,12 @@ Of course, replace ``` xyz ``` with an actual ID.
     // Or if you've left the group -- call this
     const metadata2 = await conn.groupMetadataMinimal ("abcd-xyz@g.us") 
     ```
+- To join the group using the invitation code
+    ``` ts
+    const response = await conn.acceptInvite ("xxx")
+    console.log("joined to: " + response.gid)
+    ```
+Of course, replace ``` xxx ``` with invitation code.
 
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