CONTRIBUTING.md: minor readability fixes
CONTRIBUTING.md: minor readability fixes

General fixes throughout documentation. Made sentence in bug reports section easier to understand.

authmillenon/RIOT,miri64/RIOT,josephnoir/RIOT,authmillenon/RIOT,yogo1212/RIOT,toonst/RIOT,basilfx/RIOT,jasonatran/RIOT,aeneby/RIOT,OTAkeys/RIOT,toonst/RIOT,x3ro/RIOT,x3ro/RIOT,smlng/RIOT,mtausig/RIOT,smlng/RIOT,kaspar030/RIOT,miri64/RIOT,ant9000/RIOT,mtausig/RIOT,ant9000/RIOT,kaspar030/RIOT,toonst/RIOT,miri64/RIOT,OTAkeys/RIOT,toonst/RIOT,yogo1212/RIOT,yogo1212/RIOT,smlng/RIOT,jasonatran/RIOT,OlegHahm/RIOT,authmillenon/RIOT,kYc0o/RIOT,kYc0o/RIOT,josephnoir/RIOT,authmillenon/RIOT,OTAkeys/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,mtausig/RIOT,OlegHahm/RIOT,ant9000/RIOT,mtausig/RIOT,OlegHahm/RIOT,jasonatran/RIOT,yogo1212/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,smlng/RIOT,mtausig/RIOT,miri64/RIOT,josephnoir/RIOT,x3ro/RIOT,josephnoir/RIOT,yogo1212/RIOT,OlegHahm/RIOT,x3ro/RIOT,aeneby/RIOT,basilfx/RIOT,josephnoir/RIOT,aeneby/RIOT,miri64/RIOT,basilfx/RIOT,basilfx/RIOT,ant9000/RIOT,aeneby/RIOT,ant9000/RIOT,RIOT-OS/RIOT,aeneby/RIOT,x3ro/RIOT,jasonatran/RIOT,smlng/RIOT,jasonatran/RIOT,authmillenon/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kYc0o/RIOT,kYc0o/RIOT,yogo1212/RIOT,kaspar030/RIOT,OTAkeys/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,toonst/RIOT,OTAkeys/RIOT,basilfx/RIOT