Add README

Author: blegat

README.md

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+# ArrayAD
+
+| **Build Status** |
+|:----------------:|
+| [![Build Status][build-img]][build-url] [![Codecov branch][codecov-img]][codecov-url] |
+
+Experimental addition of array support to `MOI.Nonlinear.ReverseAD`
+
+> [!WARNING]
+> This code is still very much experimental
+
+[build-img]: https://github.com/blegat/ArrayAD.jl/actions/workflows/ci.yml/badge.svg?branch=main
+[build-url]: https://github.com/blegat/ArrayAD.jl/actions?query=workflow%3ACI
+[codecov-img]: https://codecov.io/gh/blegat/ArrayAD.jl/branch/main/graph/badge.svg
+[codecov-url]: https://codecov.io/gh/blegat/ArrayAD.jl/branch/main