Explain how to test changes to haskell-mode locally

This is only the second Emacs package I've edited, and it took me a
long time to figure out how to test my changes locally the first time
around.

Author: ntc2

CONTRIBUTING.md

@@ -1,4 +1,4 @@
-Welcome nice stranger!
+# Welcome nice stranger!
 
 Haskell Mode loves contributors and we strive to go extra length to
 help out both newcomers and serial contributors.
@@ -9,3 +9,15 @@ https://github.com/haskell/haskell-mode/wiki/Contributing
 
 > When I started contributing to haskell-mode I had 0 experience in Elisp, but it was super easy to learn it.
 > -- [geraldus, 2016-01-16](https://github.com/haskell/haskell-mode/issues/1086#issuecomment-172177949)
+
+# Hacking on Haskell Mode
+
+To try out your changes to Haskell Mode without affecting your global
+Emacs installation, build Haskell Mode by running `make` here, and
+then run
+
+    emacs -L <path to this haskell-mode.git repo>
+
+from any directory you'd like to test in. This makes Emacs load
+Haskell Mode from this Git repo, instead of using any globally
+installed version you might have.