Moves minimum pg version to config, eliminates magic constant from main and improves docs.

src/PostgREST/Config.hs

@@ -8,15 +8,17 @@ It also can be used to define other middleware configuration that may be delegat
 external configuration.
 
 It currently includes a hardcoded CORS policy but this could easly be turned in configurable behaviour if needed.
+
+Other hardcoded options such as the minimum version number also belong here.
 -}
 module PostgREST.Config ( prettyVersion
                         , readOptions
                         , corsPolicy
+                        , minimumPgVersion
                         , AppConfig (..)
                         )
        where
 
-
 import           Control.Applicative
 import qualified Data.ByteString.Char8       as BS
 import qualified Data.CaseInsensitive        as CI
@@ -100,3 +102,7 @@ readOptions = customExecParser parserPrefs opts
                     <> " / create a REST API to an existing Postgres database"
                     )
     parserPrefs = prefs showHelpOnError
+
+-- | Tells the minimum PostgreSQL version required by this version of PostgREST
+minimumPgVersion :: Integer
+minimumPgVersion = 90200