Add a CONTRIBUTING.md.

CONTRIBUTING.md

@@ -0,0 +1,28 @@
+Contributing to Feldspar
+---------------------
+
+Start by reading [tibbe's style guide](https://github.com/tibbe/haskell-style-guide/blob/master/haskell-style.md).
+Current Feldspar follows most of the conventions except for the section on
+data types.
+
+Use your judgement. Having a line that is 83 characters long might be a better
+choice compared to the alternative even if it violates the style guide.
+
+1. No commits to master in your fork.
+
+1. Follow whatever code style the file uses. One consistent but bad
+   style is preferable over having eight different good styles in the
+   same file.
+
+1. No trailing whitespace.
+
+1. Run `cabal test` before committing.
+
+1. Use one commit per logical change.
+
+1. Keep the pile of small style fixes and similar in one or several 
+   separate commits.
+
+1. Make a sensible commit message.
+
+