doc: add CentOS 7 entry
doc: add CentOS 7 entry

[groonga-dev,02604]

Tested by Miyawaki and warp Kawada. Thanks!!!

naoa/mroonga,yoku0825/mroonga,mroonga/mroonga,naoa/mroonga,naoa/mroonga,naoa/mroonga,naoa/mroonga,mroonga/mroonga,yoku0825/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,yoku0825/mroonga,mroonga/mroonga,mroonga/mroonga,yoku0825/mroonga