add README badges

Author: Deric-W

README.md

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 # lambda_calculus
 
+![Tests](https://github.com/Deric-W/lambda_calculus/actions/workflows/Tests.yaml/badge.svg)
+[![codecov](https://codecov.io/gh/Deric-W/lambda_calculus/branch/main/graph/badge.svg?token=SU3982mC17)](https://codecov.io/gh/Deric-W/lambda_calculus)
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 The `lambda_calculus` package contains classes which implement basic operations of the lambda calculus.
 
 To use it, simply import the classes `Variable`, `Abstraction` and `Application` from this package