add readme to the playground (#148)

Author: ankudinov

labs/avd-playground/README.md

@@ -0,0 +1,12 @@
+# AVD Playground
+
+> [!WARNING]
+> This lab is in preview. It's fully functional, but breaking changes can happen.
+> We are working hard on building the best lab collection and your feedback is always  appreciated.
+
+Use `make start` to start the lab once AVD example inventory is unpacked into workspace and wait until lab is ready to use.
+To access any device, use `ssh <username>@<hostname>`.
+
+> Lab Credentials  
+&nbsp;&nbsp;&nbsp;&nbsp;Username: arista  
+&nbsp;&nbsp;&nbsp;&nbsp;Password: arista