Added note on lenght of the alias entry

Author: VesaJuvonen

docs/spfx/extensions/get-started/build-a-hello-world-extension.md

@@ -39,6 +39,8 @@ The next set of prompts will ask for specific information about your extension:
 
 ![Yeoman SharePoint generator prompts to create an extension solution](../../../../images/ext-yeoman-app-prompts.png)
 
+> Notice that if you use too long naming for the extension, that can cause issues. Provided entries are used to generate alias entry for the application customizer manifest json file. If alias is longer than 40 characters, you will have an exception when you are trying to serve the extension using `gulp serve --nobrowser`. You can solve this by updating the alias entry also afterwards.
+
 At this point, Yeoman will install the required dependencies and scaffold the solution files along with the **HelloWorld** extension. This might take a few minutes. 
 
 When the scaffold is complete, you should see the following message indicating a successful scaffold: