Add sprint details text field for sprints

It would be great if we could add a sprint text field we can use when we create a sprint (or anytime after creation). Ultimately, we need the ability to add these details so we can roll them up into common release notes when looking at multiple sprints in a release.

An example text would be "this sprint, we focused on improving our widget for customers as well as some other user stories related to speeding up site performance".