add nightly test badge

Author: elenafuengar

README.md

@@ -7,6 +7,8 @@
 ![PyPI - License](https://img.shields.io/pypi/l/wakis?style=flat-square&color=pink)
 ![Tokei - LOC](https://tokei.rs/b1/github/ImpedanCEI/wakis?category=code?/style=square&color=green)
 
+![Tests badge](https://github.com/impedanCEI/wakis/actions/workflows/nightly_tests_CPU.yml/badge.svg)
+
 `wakis` is a **3D Time-domain Electromagnetic solver** that solves the Integral form of Maxwell's equations using the Finite Integration Technique (FIT) numerical method. It computes the longitudinal and transverse **wake potential and beam-coupling impedance** from the simulated electric and magnetic fields. It is also a multi-purpose solver, capable of simulating planewaves interaction with nano-structures, optical diffraction, and much more!
 
 ## About