Add Aqua badge.

Author: kellertuer

Readme.md

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 `AlgorithmsInterface.jl` is a Julia package to provide a common interface to run iterative tasks.
 **Algorithm** here refers to an iterative sequence of commands, that are run until a certain stopping criterion is met.
 
-[![docs][docs-dev-img]][docs-dev-url] [![CI][ci-img]][ci-url] [![runic][runic-img]][runic-url] [![codecov][codecov-img]][codecov-url]
+[![docs][docs-dev-img]][docs-dev-url] [![CI][ci-img]][ci-url] [![runic][runic-img]][runic-url] [![codecov][codecov-img]][codecov-url] [![aqua][aqua-img]][aqua-url]
 
 [docs-dev-img]: https://img.shields.io/badge/docs-dev-blue.svg
 [docs-dev-url]: https://JuliaManifolds.github.io/AlgorithmsInterface.jl/dev/
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 [runic-img]: https://img.shields.io/badge/code_style-%E1%9A%B1%E1%9A%A2%E1%9A%BE%E1%9B%81%E1%9A%B2-black
 [runic-url]: https://github.com/fredrikekre/Runic.jl
 
+[aqua-img]: https://raw.githubusercontent.com/JuliaTesting/Aqua.jl/master/badge.svg
+[aqua-url]: https://github.com/JuliaTesting/Aqua.jl
+
 # Statement of need
 
 A first approach to algorithms is a simple for-loop for a maximum number of iterations.