Flesh out README with useful information and showcase of plots

[sadielbartholomew]

The README is very bare at the moment. We should populate it with plenty of information, for example quoting from the GitHub documentation page on READMEs:

A README is often the first item a visitor will see when visiting your repository. README files typically include information on:

What the project does
Why the project is useful
How users can get started with the project
Where users can get help with your project
Who maintains and contributes to the project

In particular I would like to include some images and links, notably showcasing some nice plots that cf-plot can make, and linking and advertising cf-python as the underlying data analysis tool.