Update day9.md
Update day9.md

Mention the update site.
Clarify that references may not be used on the right hand side of relation store rules.
basvijzendoorn/TUDelft-IN4303.github.io,TUDelft-IN4303-2016/TUDelft-IN4303-2016.github.io,victor-li/TUDelft-IN4303.github.io,basvijzendoorn/TUDelft-IN4303.github.io,victor-li/TUDelft-IN4303.github.io,TUDelft-IN4303/TUDelft-IN4303.github.io,JoopAue/TUDelft-IN4303.github.io,JoopAue/TUDelft-IN4303.github.io,JoopAue/TUDelft-IN4303.github.io,TUDelft-IN4303-2016/TUDelft-IN4303-2016.github.io,basvijzendoorn/TUDelft-IN4303.github.io,milosonator/TUDelft-IN4303.github.io,milosonator/TUDelft-IN4303.github.io,TUDelft-IN4303/TUDelft-IN4303.github.io,TUDelft-IN4303-2016/TUDelft-IN4303-2016.github.io,victor-li/TUDelft-IN4303.github.io,milosonator/TUDelft-IN4303.github.io,TUDelft-IN4303/TUDelft-IN4303.github.io