[Contributing] tweaks to the contribution chapter

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | 

Some tweaks for the changes made in #4223.

@javiereguiluz Sorry,  didn't find the time to review it in the first run.
OK, let's keep the low figure.

However, GitHub is a closed source service, which means that we don't know what they really do to count contributions. This help article explains some things, such as the cases where contributions aren't counted: https://help.github.com/articles/why-are-my-contributions-not-showing-up-on-my-profile