Add coveralls badge to README

rusqlite_migration/README.md

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-[![docs.rs](https://img.shields.io/docsrs/rusqlite_migration?style=flat-square)](https://docs.rs/rusqlite_migration) [![Crates.io](https://img.shields.io/crates/v/rusqlite_migration?style=flat-square)](https://crates.io/crates/rusqlite_migration) ![](https://img.shields.io/github/languages/code-size/cljoly/rusqlite_migration?style=flat-square) [![unsafe forbidden](https://img.shields.io/badge/unsafe-forbidden-success.svg?style=flat-square)](https://github.com/rust-secure-code/safety-dance/) [![dependency status](https://deps.rs/repo/github/cljoly/rusqlite_migration/status.svg)](https://deps.rs/repo/github/cljoly/rusqlite_migration)
+[![docs.rs](https://img.shields.io/docsrs/rusqlite_migration)](https://docs.rs/rusqlite_migration) [![Crates.io](https://img.shields.io/crates/v/rusqlite_migration)](https://crates.io/crates/rusqlite_migration) ![](https://img.shields.io/github/languages/code-size/cljoly/rusqlite_migration) [![unsafe forbidden](https://img.shields.io/badge/unsafe-forbidden-success.svg)](https://github.com/rust-secure-code/safety-dance/) [![dependency status](https://deps.rs/repo/github/cljoly/rusqlite_migration/status.svg)](https://deps.rs/repo/github/cljoly/rusqlite_migration) [![Coveralls](https://img.shields.io/coverallsCoverage/github/cljoly/rusqlite_migration)](#code-coverage)
 
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 Contributions (documentation or code improvements in particular) are welcome, see [contributing](https://cj.rs/docs/contribute/)!
 
+### Code Coverage
+
+Code coverage can only be imperfectly measured with [cargo-llvm-cov](https://crates.io/crates/cargo-llvm-cov). In particular, doc-tests are not take into account. For this reason, we exclude the `async-tokio-rusqlite` feature, which is mostly tested through doc-tests, for maintainability reasons.
+
+Full report: [Coveralls](https://coveralls.io/github/cljoly/rusqlite_migration)
+
 ## Acknowledgments
 
 I would like to thank all the contributors, as well as the authors of the dependencies this crate uses.