changed some wording in contributing instructions. Added rule about commit messages.
changed some wording in contributing instructions. Added rule about commit messages.

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