update CONTRIBUTING.md with DCO section (#4924)
update CONTRIBUTING.md with DCO section (#4924)

* update CONTRIBUTING.md with DCO section

Signed-off-by: Michal Piechowiak <9f3259e2eef1b9102300663eaa080e26c8e9c01f@gmail.com>

* Minor tweaks

Signed-off-by: Mike Allanson <4fe6e405006ae38dd82cea8575c6b1fe0de12001@gmail.com>

0x80/gatsby,fk/gatsby,0x80/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,0x80/gatsby,ChristopherBiscardi/gatsby,fk/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,ChristopherBiscardi/gatsby,mingaldrichgan/gatsby,fk/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby