Document custom quadrature rules

From https://github.com/Ferrite-FEM/Ferrite.jl/pull/482#issuecomment-1272555611 (cc @edljk)

> it could perhaps be an easier way to extend quadrature rules in Ferrite.

It is actually pretty easy to do this already -- you can construct your own quadrature rule by:

```julia
julia> weights = [2.0, 2.0];

julia> points = [Vec{2}((-0.5, 0.0)), Vec{2}((0.5, 0.0))];

julia> qr = QuadratureRule{2,RefTetrahedron,Float64}(weights, points)
QuadratureRule{2, RefTetrahedron, Float64}([2.0, 2.0], Vec{2, Float64}[[-0.5, 0.0], [0.5, 0.0]])
```

It would be nice to i) document this and ii) add a constructor `QuadratureRule{dim,shape}(weights, points)` that doesn't require specifying the element type (`Float64` in the example above).