add note for service registration in cloud (#615)

Signed-off-by: Nik Nasr <nik@restate.dev>

Author: nikrooz

docs/get_started/quickstart.mdx

@@ -642,6 +642,10 @@ SDK:
                 </TabItem>
             </Tabs>
 
+            <Admonition type="note" title="Restate Cloud">
+                When using [Restate Cloud](https://restate.dev/cloud), your service must be accessible over the public internet so Restate can invoke it. 
+                If you want to develop with a local service, you can expose it using our [tunnel](/deploy/server/cloud/#registering-restate-services-with-your-environment) feature.
+            </Admonition>
         </Step>
         <Step stepLabel="5" title="Send a request to the Greeter service">
             Invoke the service via the Restate UI playground: go to `http://localhost:9070`,  click on your service and then on playground.