Add a section on issue/MR labeling in CONTRIBUTING.md
Add a section on issue/MR labeling in CONTRIBUTING.md

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/25097

[ci skip]

jirutka/gitlabhq,iiet/iiet-git,mmkassem/gitlabhq,jirutka/gitlabhq,t-zuehlsdorff/gitlabhq,dplarson/gitlabhq,jirutka/gitlabhq,darkrasid/gitlabhq,mmkassem/gitlabhq,dreampet/gitlab,dreampet/gitlab,stoplightio/gitlabhq,mmkassem/gitlabhq,jirutka/gitlabhq,dplarson/gitlabhq,darkrasid/gitlabhq,dreampet/gitlab,darkrasid/gitlabhq,iiet/iiet-git,dplarson/gitlabhq,t-zuehlsdorff/gitlabhq,axilleas/gitlabhq,dreampet/gitlab,t-zuehlsdorff/gitlabhq,htve/GitlabForChinese,darkrasid/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,htve/GitlabForChinese,t-zuehlsdorff/gitlabhq,axilleas/gitlabhq,stoplightio/gitlabhq,htve/GitlabForChinese,iiet/iiet-git,axilleas/gitlabhq,stoplightio/gitlabhq,dplarson/gitlabhq,htve/GitlabForChinese,axilleas/gitlabhq,iiet/iiet-git