Add note in PGF2 documentation about risk for integer overflow.

Closes #109

src/runtime/haskell-bind/PGF2/Expr.hsc

@@ -140,7 +140,9 @@ unStr (Expr expr touch) =
               touch
               return (Just s)
 
--- | Constructs an expression from an integer literal
+-- | Constructs an expression from an integer literal.
+-- Note that the C runtime does not support long integers, and you may run into overflow issues with large values.
+-- See [here](https://github.com/GrammaticalFramework/gf-core/issues/109) for more details.
 mkInt :: Int -> Expr
 mkInt val =
   unsafePerformIO $ do