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 Job ID Name Coverage
  Test
passed #2570
Go-py2.7

00:01:32

passed #2571
Go-py3.4

00:01:34

passed #2572
Go-py3.5

00:01:36

passed #2573
Go-py3.6

00:01:35