refine contributing text in several places

runtox.py

@@ -0,0 +1,10 @@
+#!/usr/bin/env python
+
+import subprocess
+import sys
+
+if __name__ == "__main__":
+    subprocess.call(["tox",
+                     "-i", "ALL=https://devpi.net/hpk/dev/",
+                     "--develop",] + sys.argv[1:])
+