Populate README with useful information and screenshot previews

[sadielbartholomew]

As with NCAS-CMS/cf-plot#27 (comment), we should add plenty of information to the README for cf-view. In this case, instead of an example gallery, we should include at least one (ideally more, to demonstrate multiple views) screenshots of the GUI in active use.

For now, I will use the one Andy has included in the homepage of the documentation, but I will go on to try to take other screenshots and add those in.