Update README.md

Author: eunwoo1104

README.md

@@ -22,7 +22,7 @@ async def _test(ctx: SlashContext):
 bot.run("discord_token")
 ```
 
-Cog:
+Cog(Not Recommended):
 ```py
 import discord
 from discord.ext import commands
@@ -36,6 +36,8 @@ class Slash(commands.Cog):
         self.slash = SlashCommand(bot, override_type=True)
         # Cog is only supported by commands ext, so just skip checking type.
 
+        # Make sure all commands should be inside `__init__`
+        # or some other functions that can put commands.
         @self.slash.slash(name="test")
         async def _test(ctx: SlashContext):
             await ctx.send(content="Hello, World!")