Add caveat note about Message.mention_everyone
Add caveat note about Message.mention_everyone

lgkern/PriestPy,rapptz/discord.py,Theoretical/UselessBot,khazhyk/discord.py,variablehair/Eggplantato,Rapptz/discord.py,jhgg/discord.py,Harmon758/discord.py,imayhaveborkedit/discord.py,Harmon758/discord.py,mgardne8/discord.py