Update contributing.md
Update contributing.md

I'm testing contributions and added a sentence about where to find the main chapter.
aci-ref/facilitation_best_practices,aci-ref/facilitation_best_practices,aci-ref/aci-ref.github.io,aci-ref/facilitation_best_practices,devbioinfoguy/facilitation_best_practices,aci-ref/aci-ref.github.io,devbioinfoguy/facilitation_best_practices,aci-ref/aci-ref.github.io