Inaccurate description of the readme for ldap_group in the readme.

[zemla44]

While developing the help charts for deploying Сerebro, I encountered an issue where authentication kept falling with the error "invalid password or name." After struggling with this problem for some time, I finally discovered what was causing it, and group-based authorization via LDAP started working for me. In this file—https://github.com/lmenezes/cerebro/blob/main/examples/open_ldap_group/README.md the variable LDAP_GROUP_BASE_DN isn't described. Initially, I duplicated or substitute values for connecting to the LDAP server, but eventually, leaving the value empty made it work. However, here—https://github.com/lmenezes/cerebro/blob/main/conf/application.conf#L50—it states that if LDAP_GROUP_BASE_DN is empty, it uses LDAP_BIND_DN.

I request that you updated the README by adding a description for this variable, as it's not obvious and can only be understood through trial and error.