Update README.md (#122)

Author: TSGut

README.md

@@ -100,4 +100,14 @@ julia> Δ = (D*S)'*(D*S) # weak laplacian corresponding to inner products of wei
  ⋮                                         ⋮                         ⋱  
 ```
 
+## Related Packages
+
+**[HarmonicOrthogonalPolynomials.jl](https://github.com/JuliaApproximation/HarmonicOrthogonalPolynomials.jl)**: adds support for spherical harmonic expansions and harmonic polynomials in balls.
+
+**[MultivariateOrthogonalPolynomials.jl](https://github.com/JuliaApproximation/MultivariateOrthogonalPolynomials.jl)**: adds support for multivariate orthogonal polynomials on disks, spheres, triangles, and other simple geometries
+
+**[SemiclassicalOrthogonalPolynomials.jl](https://github.com/JuliaApproximation/SemiclassicalOrthogonalPolynomials.jl)**: adds support for semiclassical orthogonal polynomials such as Semiclassical Jacobi polynomials with weights `x^a * (1-x)^b * (t-x)^c`.
+
+**[PiecewiseOrthogonalPolynomials.jl](https://github.com/JuliaApproximation/PiecewiseOrthogonalPolynomials.jl)**: adds support for piecewise orthogonal polynomials which can be used in p-FEM.
+