Add note for edit_role about custom RGB colours.
Add note for edit_role about custom RGB colours.

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