message: Add info in c api readme in the testing section
diff: @@ -105,6 +105,10 @@ int main()
  # Testing
  +Tests are run using the release build of the library. If you make
  +changes or compile with non-default features, please ensure you
  +rebuild in release mode for the tests to see the changes.
  +
  The tests can be run via `cargo test`, such as:
  ```sh
  