Add module doc for Plugins.

This was requested in #15650.

compiler/main/Plugins.hs

 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE CPP #-}
+
+-- | Definitions for writing /plugins/ for GHC. Plugins can hook into
+-- several areas of the compiler. See the 'Plugin' type. These plugins
+-- include type-checker plugins, source plugins, and core-to-core plugins.
+
 module Plugins (
       -- * Plugins
       Plugin(..)