Comparison to GraphNeuralNetworks.jl

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Motivation and description

I'm looking for a GNN package and found both GraphNeuralNetworks.jl and GeometricFlux.jl.
A summary of the differences between the two packages in the README would be helpful when trying to decide which one to use.

See also JuliaGraphs/GraphNeuralNetworks.jl#502.

Possible Implementation

• Update README with a reference to GraphNeuralNetworks.jl and a short comparison of design goals / features and possibly performance