Add code snippet to readme

[ddundo]

I think a great thing about Animate is that we can adapt a mesh with only a few lines of code, without requiring much user knowledge/effort, e.g.

P1ten = TensorFunctionSpace(mesh, "CG", 1)
metric = RiemannianMetric(P1ten)
metric.set_parameters(...)
metric.compute_hessian(f)
metric.normalise()
adapted_mesh = adapt(mesh, metric)

I think it would be great to show this off in the readme.

[jwallwork23]

Yeah that'd be good!