Remove "official merge window" from CONTRIBUTING.md [ci skip]
Remove "official merge window" from CONTRIBUTING.md [ci skip]

We should not discourage contributors from submitting merge requests
anytime they want. Our process for reviewing community contributions
no longer depends on the release schedule.
LUMC/gitlabhq,darkrasid/gitlabhq,dreampet/gitlab,iiet/iiet-git,jirutka/gitlabhq,iiet/iiet-git,t-zuehlsdorff/gitlabhq,t-zuehlsdorff/gitlabhq,dreampet/gitlab,dplarson/gitlabhq,htve/GitlabForChinese,t-zuehlsdorff/gitlabhq,stoplightio/gitlabhq,dreampet/gitlab,darkrasid/gitlabhq,dplarson/gitlabhq,openwide-java/gitlabhq,axilleas/gitlabhq,dreampet/gitlab,iiet/iiet-git,t-zuehlsdorff/gitlabhq,mmkassem/gitlabhq,jirutka/gitlabhq,darkrasid/gitlabhq,mmkassem/gitlabhq,dplarson/gitlabhq,jirutka/gitlabhq,LUMC/gitlabhq,htve/GitlabForChinese,stoplightio/gitlabhq,openwide-java/gitlabhq,jirutka/gitlabhq,stoplightio/gitlabhq,iiet/iiet-git,axilleas/gitlabhq,stoplightio/gitlabhq,htve/GitlabForChinese,axilleas/gitlabhq,LUMC/gitlabhq,openwide-java/gitlabhq,mmkassem/gitlabhq,mmkassem/gitlabhq,axilleas/gitlabhq,dplarson/gitlabhq,htve/GitlabForChinese,darkrasid/gitlabhq,LUMC/gitlabhq,openwide-java/gitlabhq