Improve Contribution Guide

[felipecosta09]

Problem to solve

During PR code review, multiple times changes are requested from the reviewer to jump one line after each end of the file that stores code. I'm assuming this is because of the POSIX format(Thanks @andresark for explaining to me what that is), but the lack of documentation and awareness about this standard makes the PR review very long. I understand the importance of using it, but the contributor should have this information beforehand so it can make it right from the first time.

Proposed solution

Update the Contribution Guide with these requirements so the contributor can make the PR already with these formats so we can make the PR review more straightforward and fast.