Add instructions about required "setup" function on cog extensions

[EmoonX]

As I was getting myself acquainted with cogwatch (and Cogs themselves), unfortunately, I lost quite some time dealing with extension-related errors upon trying to reload a cog file; until finding out I needed to add the following function at the bottom of it:

def setup(bot: commands.Bot):
    '''Add cog every time extension (module) is (re)loaded.'''
    bot.add_cog(MyCog(bot))

However, there is no explicit reference to the Extension concept in cogwatch tutorial, even though discord.py extensions page mentions hot-reloading. Since it seems to be a lower-level concept that many users that arrive here aren't aware of (myself included), I think it might be interesting to add tutorial instructions regarding these points (especially the needed setup function). 🙂

[robertwayne]

Thanks for the feedback!

I've added a complete example to the examples directory that should help guide developers unfamiliar with the cog concept in the future!