Add a note about using ctx.respond

Author: hypergonial

examples/app_commands/slash_basic.py

@@ -14,6 +14,8 @@
 async def hello(ctx):
     """Say hello to the bot"""  # the command description can be supplied as the docstring
     await ctx.respond(f"Hello {ctx.author}!")
+    # Please note that you MUST either use ctx.respond() or ctx.defer() in your
+    # slash command code, otherwise the interaction will fail.
 
 
 @bot.slash_command(