Add Headless CMS learning resource

[gregsnewby]

Hi there! I wanted to share an article outlining all the details of Headless CMSs. I thought it would be a good additional resource for the handbook since it covers things like:

• A brief history of the Content Management System
• What does “headless” mean anyway?
• What are the main differences between headless and traditional CMSs?
• What applications can you create with a headless cms?
• Examples of actual sites on the web using headless CMSs

The link is here: https://buttercms.com/blog/headless-cms-learn-the-what-why-and-how

[lumettak]

Hi @codylindley @ryanZiegler! I just wanted to follow up on this and get your thoughts. I'd be happy to open a PR if required. Thanks! Kristen