Add Hercules CI badge

Author: szg251

README.md

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 # Plutus Ledger API
 
+[![Hercules CI](https://hercules-ci.com/api/v1/site/github/account/mlabs-haskell/project/plutus-ledger-api-rust/badge)](https://hercules-ci.com/github/mlabs-haskell/plutus-ledger-api-rust/status)
+
 ## Packages
 
 - [plutus-ledger-api](./plutus-ledger-api/) - Plutus Ledger types and utilities