Hello there. This had me puzzled at first too. When you write the hub, do a couple of paragraphs, then save them. Look to the right of the page, and click on "text." This adds a new text capsule, which has the option of adding a subtitle.
It's a good idea to use several text capsules because you can then place photos within the hub, either between text capsules, or making them smaller and putting them to one side of the text. In edit, click on the arrows to move the pics up/down/shrink/back to full size.
Hope this makes sense. The forums are a good place to ask for advice, that's where I learned how to do this.