Add quick start section to readme

test/Database/PostgreSQL/OpiumSpec.hs

@@ -1,5 +1,6 @@
 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE DuplicateRecordFields #-}
+{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE TypeApplications #-}
 
@@ -38,6 +39,13 @@ data ManyFields = ManyFields
 
 instance Opium.FromRow ManyFields where
 
+data ScoreByAge = ScoreByAge
+  { m :: Double
+  , t :: Double
+  } deriving (Eq, Generic, Show)
+
+instance Opium.FromRow ScoreByAge where
+
 isLeft :: Either a b -> Bool
 isLeft (Left _) = True
 isLeft _ = False
@@ -125,3 +133,7 @@ spec = do
     it "Fails for the wrong column type" $ \conn -> do
       rows <- Opium.fetch_ @Person conn "SELECT 'quby' AS name, 'indeterminate' AS age"
       rows `shouldBe` Left (Opium.ErrorInvalidOid "age" $ LibPQ.Oid 25)
+
+    it "Works for the readme regression example" $ \conn -> do
+      rows <- Opium.fetch_ @ScoreByAge conn "SELECT regr_intercept(score, age) AS t, regr_slope(score, age) AS m FROM person"
+      rows `shouldSatisfy` \case { (Right [ScoreByAge _ _]) -> True; _ -> False }