Add default_branch attr to Project API payload in docs.
Add default_branch attr to Project API payload in docs.

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