Add more comparisons to other packages

[kbarros]

The landing-page Readme has a comparison table for marketing purposes, but it is limited due to space constraints. Other interesting packages include:

• https://github.com/UppASD (Tons of interesting research: First principles determination of exchange, coupled atom-spin dynamics, various transport measurements, support for disordered systems, in development since 2008, Python, has a companion book).

• http://micromagnetics.org/magnum.fe/index.html (Support for micromagnetic simulations on meshes.)