Add usage section #6
Description
In index.md, when one package uses MathematicalSets, add a subsection like the following:
## Usage
The following packages implement the `MathematicalSets.AbstractSet` interface:
|Library|Description|
|-------|------------|
|[LazySets](https://github.com/JuliaReach/LazySets.jl)|A Julia package for calculus with convex sets|
|[Polyhedra](https://github.com/JuliaPolyhedra/Polyhedra.jl)|Polyhedral Computation Interface|
|[SemialgebraicSets](https://github.com/JuliaAlgebra/SemialgebraicSets.jl)|Extension of MultivariatePolynomials to semialgebraic sets|
|[SetProg](https://github.com/blegat/SetProg.jl)|Set Programming with JuMP|