Add breadcrumbs divider option to readme

Currently, there is an option to set up breadcrumbs divider. Unfortunately, it's not stated in the README.md.
This commit adds it to readme so that new users don't have to check gem sources to configure divider (some users like me would like this option, especially after transition from [breadcrumbs_on_rails] (https://github.com/weppos/breadcrumbs_on_rails)).