add contributing guide for pull requests

Author: isaacs

CONTRIBUTING.md

@@ -0,0 +1,14 @@
+**NO PATCHES WITHOUT A TEST**
+
+**TEST MUST PASS WITH THE PATCH.**
+
+**TEST MUST FAIL WITHOUT THE PATCH.**
+
+**NO EXCEPTIONS.**
+
+# EVERY PULL REQUEST MUST HAVE A TEST.
+
+Seriously.  This is a very strict rule, and I will not bend it for any
+patch, no matter how minor.
+
+Write a test.