add class docstring to downloader

idk what to say rly
um hi all nice to be back after about 6 months
pls no hate my PRs
apoligies if i get in the way
if you read this far then you deserve an achievement um i dunno what err how about the achievement of unboredness

redbot/cogs/downloader/downloader.py

@@ -36,6 +36,16 @@ DEPRECATION_NOTICE = _(
 
 @cog_i18n(_)
 class Downloader(commands.Cog):
+    """Install community cogs from Cog Creators.
+
+    Community cogs, also called third party cogs, are not included
+    in the default Red install. Cogs come in repositories. Repos are
+    groups of cogs by one creator you can install.
+
+    You always need to add a repository using the `[p]repo` command
+    before you can install cogs from the repo.
+    """
+
     def __init__(self, bot: Red):
         super().__init__()
         self.bot = bot