Add CONTRIBUTING.md
Add CONTRIBUTING.md

The file mostly contains adapted contents @theska wrote for our website and the great CONTRIBUTING.md of phpDocumentor (https://github.com/phpDocumentor/phpDocumentor2/blob/v2.8.5/CONTRIBUTING.md)

TakamiChie/Pico,ondrejd/Pico,TakamiChie/Pico,dav-m85/Pico,lukasz-jakub-adamczuk/tech-jump-guide,dav-m85/Pico,picocms/Pico,PhrozenByte/Pico,picocms/Pico,ondrejd/Pico,theshka/Pico,theshka/Pico,fabianacero/fabianacerog,smcdougall/Pico,smcdougall/Pico,TakamiChie/Pico,lukasz-jakub-adamczuk/tech-jump-guide,TakamiChie/Pico,fabianacero/fabianacerog,dav-m85/Pico,PhrozenByte/Pico,ondrejd/Pico,lukasz-jakub-adamczuk/tech-jump-guide,smcdougall/Pico,fabianacero/fabianacerog,PhrozenByte/Pico,theshka/Pico,picocms/Pico