Slight updates to CONTRIBUTING

- most issues have been closed in other commits
4 jobs for 141-contributing-update in 1 minute and 36 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed Go-py2.7 #2570

00:01:32

passed Go-py3.4 #2571

00:01:34

passed Go-py3.5 #2572

00:01:36

passed Go-py3.6 #2573

00:01:35