Update release_notes.md
Update release_notes.md

Adding "Send to All Contacts" update to Marketing Campaigns.
Whatthefoxsays/docs,ksigler7/docs,ksigler7/docs,sendgrid/docs,sendgrid/docs,katieporter/docs,sendgrid/docs,Whatthefoxsays/docs,katieporter/docs,katieporter/docs,Whatthefoxsays/docs,ksigler7/docs,katieporter/docs,katieporter/docs