Added Github action badge to README

README.md

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 # Uno
 
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    *A modern, modular solver for nonlinearly constrained nonconvex optimization*
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 Uno (Unifying Nonlinear Optimization) is a C++ library that unifies methods for solving nonlinearly constrained optimization problems of the form:
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 [David Kiessling](https://github.com/david0oo) implemented the funnel method.  
 [Silvio Traversaro](https://github.com/traversaro) and [Alexis Montoison](https://github.com/amontoison) contributed to the CMakeLists.
 
+![Unit tests on Ubuntu workflow](https://github.com/cvanaret/Uno/actions/workflows/unit-tests-ubuntu.yml/badge.svg)
 
 ## Unifying nonlinearly constrained nonconvex optimization