Added CoC section
Added CoC section

This PR:    - Adds a CoC section into contribution doc page.
Related to #1881
Author: Ivan Ogasawara <ivan.ogasawara@gmail.com>

Closes #1882 from xmnlab/add_numfocus_coc and squashes the following commits:

2ede708 [Ivan Ogasawara] Added CoC section
cloudera/ibis,ibis-project/ibis,cpcloud/ibis,ibis-project/ibis,cpcloud/ibis,cloudera/ibis,ibis-project/ibis,ibis-project/ibis,cpcloud/ibis,cpcloud/ibis,cloudera/ibis