Update docs.md
Update docs.md

be a bit more descriptive regarding the sass compiler and how to make it run for beginners like me :) . It took a while to figure out what is going on since the little change to custom.css had no results without sass installed and running in the folder to watch the changes. Originally i thought it would be done automatically but guess not.
getgrav/grav-learn,Quy/grav-learn,Quy/grav-learn,getgrav/grav-learn,miljan-aleksic/grav-learn,Quy/grav-learn,getgrav/grav-learn,miljan-aleksic/grav-learn,getgrav/grav-learn,miljan-aleksic/grav-learn,Quy/grav-learn,miljan-aleksic/grav-learn