[1.5.x] Fixed #21047 -- Added CLA mesage on the new contributor advice doc
[1.5.x] Fixed #21047 -- Added CLA mesage on the new contributor advice doc

Backport of 93dd31cadf from master

ccn-2m/django,bliti/django-nonrel-1.5,ccn-2m/django,bliti/django-nonrel-1.5,ccn-2m/django,imtapps/django-imt-fork,bliti/django-nonrel-1.5,ccn-2m/django,imtapps/django-imt-fork,imtapps/django-imt-fork