Contributing Guidelines: Add a commit msg one-liner length limit

Refering to: scality/Arsenal#9

Most guidelines over the commit message enforce a really tight one-liner (52 chars), to fit into a web/text UI displaying only the one-liners.

We need to discuss how long are the commit messages one-liners that we're going to go for within our project.

As said earlier, I'm fine with a soft limit at 50cols (i.e. something we should strive for, but not a hard rule) and a hard limit at 72 cols (If you need more, the commit is probably not summarized nor atomic).

But I don't think a lot of people in @scality/team-ironman saw this.

[adrienverge]

+1 with Michael. The 1st line 50 / next ones 72 max is a convention used everywhere.

A bit of history: http://stackoverflow.com/questions/2290016/git-commit-messages-50-72-formatting