Add a HACKING file

Seems people do not know that the source guide exists.

Add a HACKING file
affe481c · Duncan Coutts · a7adb799 · affe481c
Commit affe481c authored 15 years ago by Duncan Coutts
--- a/HACKING
+++ b/HACKING
+If you want to hack on Cabal, don't be intimidated!
+
+Read the guide to the source code:
+  http://hackage.haskell.org/trac/hackage/wiki/SourceGuide
+
+There are other resources listed on the dev wiki:
+  http://hackage.haskell.org/trac/hackage/
+
+In particular, the open tickets and the cabal-devel mailing list
+which is a good place to ask questions.