CONTRIBUTING: move the 'writing documentation' section
CONTRIBUTING: move the 'writing documentation' section

Because the *working with git* section is quite long, it obscures the
*writing documentation* section. Move it to before the *working with
git* section.

OTAkeys/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,OTAkeys/RIOT,kYc0o/RIOT,miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kaspar030/RIOT,authmillenon/RIOT,ant9000/RIOT,jasonatran/RIOT,OlegHahm/RIOT,kYc0o/RIOT,jasonatran/RIOT,authmillenon/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kaspar030/RIOT,OTAkeys/RIOT,kYc0o/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,miri64/RIOT,jasonatran/RIOT,ant9000/RIOT,jasonatran/RIOT,kaspar030/RIOT,miri64/RIOT,ant9000/RIOT,authmillenon/RIOT,kYc0o/RIOT,ant9000/RIOT,authmillenon/RIOT,miri64/RIOT,RIOT-OS/RIOT,miri64/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,ant9000/RIOT