Add onsite-messaging and engagement tools

[hengwoon]

On-site messaging is increasingly important to capture potential leads and customers as they land on your site as well as proactively engaging them (for example asking them if they have any questions when they are on the pricing page, or customizing a welcome message when visiting the landing page from a specific marketing channel)

[karllhughes]

Thanks @hengwoon! I've added you to the contributors list. Feel free to make another PR or issue if you see ways we can improve the checklist!