[ci skip] update README.md

README.md

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-[![build](https://github.com/MadNLP/MadNLP.jl/workflows/build/badge.svg?branch=dev%2Fgithub_actions)](https://github.com/MadNLP/MadNLP.jl/actions?query=workflow%3Abuild) [![codecov](https://codecov.io/gh/MadNLP/MadNLP.jl/branch/master/graph/badge.svg?token=MBxH2AAu8Z)](https://codecov.io/gh/MadNLP/MadNLP.jl) [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.5825776.svg)](https://doi.org/10.5281/zenodo.5825776)
+| **Documentation** | **Build Status** | **Coverage** | **DOI** |
+|:-----------------:|:----------------:|:----------------:|:----------------:|
+| [![doc](https://img.shields.io/badge/docs-dev-blue.svg)](https://madnlp.github.io/MadNLP.jl/dev) | [![build](https://github.com/MadNLP/MadNLP.jl/workflows/build/badge.svg?branch=dev%2Fgithub_actions)](https://github.com/MadNLP/MadNLP.jl/actions?query=workflow%3Abuild) | [![codecov](https://codecov.io/gh/MadNLP/MadNLP.jl/branch/master/graph/badge.svg?token=MBxH2AAu8Z)](https://codecov.io/gh/MadNLP/MadNLP.jl) | [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.5825776.svg)](https://doi.org/10.5281/zenodo.5825776) |
 
 MadNLP is a [nonlinear programming](https://en.wikipedia.org/wiki/Nonlinear_programming) (NLP) solver, purely implemented in [Julia](https://julialang.org/). MadNLP implements a filter line-search algorithm, as that used in [Ipopt](https://github.com/coin-or/Ipopt). MadNLP seeks to streamline the development of modeling and algorithmic paradigms in order to exploit structures and to make efficient use of high-performance computers.