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

[ghost]

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.

[ghost]

I'm fine with a soft limit at 52cols (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).