Adding myself as co-maintainer to docker-haskell

as per https://github.com/haskell/docker-haskell/issues/5#issuecomment-551191030

library/haskell

@@ -1,5 +1,6 @@
 Maintainers: Peter Salvatore <[email protected]> (@psftw),
              Christopher Biscardi <[email protected]> (@ChristopherBiscardi)
+             Herbert Valerio Riedel <[email protected]> (@hvr)
 GitRepo: https://github.com/haskell/docker-haskell
 
 Tags: 8.8.3, 8.8, 8, latest