Haskell - add custom slim section

haskell/variant-slim.md

@@ -0,0 +1,5 @@
+## `%%IMAGE%%:<version>-slim`
+
+This image does not contain the common packages contained in the default tag and only contains the minimal packages needed to run `ghc`, `cabal-install` and `stack`. In addition, [profiling support](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/profiling.html) is not included which saves ~ 700MB of space.
+
+If image size is of concern, we recommend using the slim images.