Tweak the coding conventions section.

HACKING.md

@@ -134,8 +134,9 @@ Coding Conventions
 
 Use spaces, not tabs. Use lines no longer than 80 characters. If you modify a
 file, please follow the style conventions used in that file. When you add a new
-top-level definition, please also add a Haddock comment. Use of GHC extensions
-is allowed (except Template Haskell), provided that the dependencies policy is
+top-level definition, please also add a Haddock comment. Use explicit import
+lists for third-party and standard library imports. Use of GHC extensions is
+allowed (except Template Haskell), provided that the dependencies policy is
 respected. In general, try to adhere to [this style guide][guide].
 
 [guide]: https://github.com/tibbe/haskell-style-guide/blob/master/haskell-style.md