Update the "latest update" on Roadmap page

[SarahPentland]

Roadmap page - latest update

Perpetual Web Browser Personas & Usecases
The Perpetual Web is the first end-user focused milestone on the way to the Beta launch. It will showcase and enable the perpetual, versioned, nature of all public data. The app at the heart of this is the Safe Browser. Take a sneak peak at the design process for this milestone over on GitHub.

https://github.com/maidsafe/perpetual-web-browser