Update README.md to include docs badge

Author: nHackel

README.md

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 [![codecov](https://codecov.io/github/JuliaImageRecon/LinearOperatorCollection.jl/graph/badge.svg?token=MEjup4lqjO)](https://codecov.io/github/JuliaImageRecon/LinearOperatorCollection.jl)
 
+[![](https://img.shields.io/badge/docs-latest-blue.svg)](https://JuliaImageRecon.github.io/RegularizedLeastSquares.jl/latest)
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 ## Purpose
 
 This package contains a collection of linear operators that are in particular useful for multi-dimensional signal and image processing tasks. All operators are build using the LinearOperators.jl base type and derive from `AbstractLinearOperator`. For example this package