Document that we don't want pushed commits to be rebased and give a rationale for many small MR's policy.
Merge branch 'small-contributions-no-rebase' into 'master'

Small contributions no rebase

Based on https://github.com/gitlabhq/gitlabhq/pull/7067#issuecomment-45060241

/cc @dzaporozhets
Merge branch 'master' of gitlab.com:gitlab-org/gitlab-ce