docs: Add test and coverage badges

README.md

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 # Nurse Scheduling System (護理排班系統)
 
+[![tests](https://img.shields.io/github/actions/workflow/status/j3soon/nurse-scheduling/test-core.yaml?label=tests)](https://github.com/j3soon/nurse-scheduling/actions/workflows/test-core.yaml)
+[![codecov](https://codecov.io/github/j3soon/nurse-scheduling/graph/badge.svg?token=DPOvtAW1k2)](https://codecov.io/github/j3soon/nurse-scheduling)
+
 The nurse scheduling (or employee scheduling) problem is a well-known problem in the field of operations research (OR) and can be (approximately) solved efficiently by constrained optimization.
 
 However, the constraints vary significantly across hospitals or wards. Therefore, in practice, the problem is often solved by hand or with the help of Excel, which is really time-consuming. The entire process requires several hours or even more than ten hours, depending on the problem complexity (e.g., multiple wards).