[master] added link to contributor's guide

README

@@ -9,5 +9,8 @@ The developers website is at http://kea.isc.org/
 Limitations and known issues with this DHCP release can be found
 at http://kea.isc.org/wiki/KeaKnownIssues
 
+If you are thinking about contributing a patch, please read this first:
+http://kea.isc.org/wiki/ContributorsGuide
+
 The Kea mailing lists are available via
 https://lists.isc.org/