extra: give hledger-rewrite.hs a stack script header

and some guidance for compiling or interpreting it.
This may be helpful for getting started, although the suggested
commands still require that you're in the hledger source tree.

extra/hledger-rewrite.hs

@@ -1,5 +1,13 @@
-#!/usr/bin/env runhaskell
--- {-# LANGUAGE CPP #-}
+#!/usr/bin/env stack
+{- stack runghc --verbosity info
+  --package hledger-lib
+  --package hledger
+  --package megaparsec
+  --package text
+-}
+-- To compile this script:  cd hledger; stack ghc extra/hledger-rewrite.hs
+-- To run it "interpreted": cd hledger; extra/hledger-rewrite.hs ARGS
+
 {-|
 hledger-rewrite [PATTERNS] --add-posting "ACCT  AMTEXPR" ...