Add fat prelude documentation [ci skip]

Cabal/Distribution/Compat/Prelude.hs

@@ -10,6 +10,11 @@
 #define MINVER_base_46 (__GLASGOW_HASKELL__ >= 706)
 #endif
 
+-- | This module does two things:
+--
+-- * Acts as a compatiblity layer, like @base-compat@.
+--
+-- * Provides commonly used imports.
 module Distribution.Compat.Prelude (
     -- * Prelude
     --

README.md

@@ -162,6 +162,9 @@ Conventions
   must be able to download and install all of the dependencies.
   (This is also checked by CI!)
 
+* `Cabal` has own compat and somehow fat prelude, in
+  `Distribution.Compat.Prelude`, Use it for new modules as well.
+
 We like [this style guide][guide].
 
 [guide]: https://github.com/tibbe/haskell-style-guide/blob/master/haskell-style.md