added more git instructions to the contributing page.
added more git instructions to the contributing page.

ces0712/mtools,aerrami/mtools,corymintz/mtools,ces0712/mtools,tomzhang/mtools,yaowenqiang/mtools,yaowenqiang/mtools,parkhyeonghwa/mtools,aerrami/mtools,tomzhang/mtools,rueckstiess/mtools,parkhyeonghwa/mtools,corymintz/mtools,gormanb/mtools,yaowenqiang/mtools,parkhyeonghwa/mtools,devkev/mtools,tomzhang/mtools,gormanb/mtools,aerrami/mtools,devkev/mtools,gormanb/mtools,corymintz/mtools,devkev/mtools,ces0712/mtools,rueckstiess/mtools