Add EOL note to time compat module.

Because time 1.4 is a boot library for GHC 7.8, we will support the
compatibility module as long as we support 7.8. But we should be clear
about when we will no longer need it.

src/Text/Pandoc/Compat/Time.hs

@@ -1,4 +1,16 @@
 {-# LANGUAGE CPP #-}
+
+{-
+This compatibility module is needed because, in time 1.5, the
+`defaultTimeLocale` function was moved from System.Locale (in the
+old-locale library) into Data.Time.
+
+We support both behaviors because time 1.4 is a boot library for GHC
+7.8. time 1.5 is a boot library for GHC 7.10.
+
+When support is dropped for GHC 7.8, this module may be obsoleted.
+-}
+
 #if MIN_VERSION_time(1,5,0)
 module Text.Pandoc.Compat.Time (
   module Data.Time