Add CONTRIBUTING.md for contributing guidelines

This also lets us hook into the slick interface that GitHub provides:
https://github.com/blog/1184-contributing-guidelines

CONTRIBUTING.md

@@ -0,0 +1,29 @@
+## Contributing to pq
+
+`pq` has a backlog of pull requests, but contributions are still very
+much welcome. You can help with patch review, submitting bug reports,
+or adding new functionality. There is no formal style guide, but
+please conform to the style of existing code and general Go formatting
+conventions when submitting patches.
+
+### Patch review
+
+Help review existing open pull requests by commenting on the code or
+proposed functionality.
+
+### Bug reports
+
+We appreciate any bug reports, but especially ones with self-contained
+(doesn't depend on code outside of pq), minimal (can't be simplified
+further) test cases. It's especially helpful if you can submit a pull
+request with just the failing test case (you'll probably want to
+pattern it after the tests in
+[conn_test.go](https://github.com/lib/pq/blob/master/conn_test.go).
+
+### New functionality
+
+There are a number of pending patches for new functionality, so
+additional feature patches will take a while to merge. Still, patches
+are generally reviewed based on usefulness and complexity in addition
+to time-in-queue, so if you have a knockout idea, take a shot. Feel
+free to open an issue discussion your proposed patch beforehand.