Add --maturity-period to README

### Clear and concise description of the problem `--maturity-period` is not currently listed in the documentation ### Suggested solution Adding it in the README #205 ### Alternative _No response_ ### Additional context _No response_ ### Validations - [x] Follow our [Code of Conduct](https://github.com/antfu/.github/blob/main/CODE_OF_CONDUCT.md) - [x] Read the [Contributing Guide](https://github.com/antfu/contribute). - [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.