Document how to track custom events
Document how to track custom events

Fixes gitlab-org/gitlab-ce#22070

[ci skip]

dreampet/gitlab,darkrasid/gitlabhq,mmkassem/gitlabhq,htve/GitlabForChinese,openwide-java/gitlabhq,daiyu/gitlab-zh,screenpages/gitlabhq,openwide-java/gitlabhq,allysonbarros/gitlabhq,jirutka/gitlabhq,dplarson/gitlabhq,allysonbarros/gitlabhq,openwide-java/gitlabhq,dplarson/gitlabhq,t-zuehlsdorff/gitlabhq,SVArago/gitlabhq,jirutka/gitlabhq,shinexiao/gitlabhq,icedwater/gitlabhq,axilleas/gitlabhq,t-zuehlsdorff/gitlabhq,shinexiao/gitlabhq,shinexiao/gitlabhq,dreampet/gitlab,mmkassem/gitlabhq,dplarson/gitlabhq,LUMC/gitlabhq,mmkassem/gitlabhq,LUMC/gitlabhq,allysonbarros/gitlabhq,stoplightio/gitlabhq,SVArago/gitlabhq,icedwater/gitlabhq,htve/GitlabForChinese,mmkassem/gitlabhq,jirutka/gitlabhq,t-zuehlsdorff/gitlabhq,daiyu/gitlab-zh,screenpages/gitlabhq,darkrasid/gitlabhq,darkrasid/gitlabhq,iiet/iiet-git,stoplightio/gitlabhq,iiet/iiet-git,icedwater/gitlabhq,darkrasid/gitlabhq,axilleas/gitlabhq,openwide-java/gitlabhq,axilleas/gitlabhq,dreampet/gitlab,dplarson/gitlabhq,dreampet/gitlab,LUMC/gitlabhq,stoplightio/gitlabhq,htve/GitlabForChinese,SVArago/gitlabhq,daiyu/gitlab-zh,LUMC/gitlabhq,screenpages/gitlabhq,screenpages/gitlabhq,daiyu/gitlab-zh,htve/GitlabForChinese,iiet/iiet-git,allysonbarros/gitlabhq,iiet/iiet-git,icedwater/gitlabhq,t-zuehlsdorff/gitlabhq,stoplightio/gitlabhq,shinexiao/gitlabhq,axilleas/gitlabhq,jirutka/gitlabhq,SVArago/gitlabhq