Please explain in the docs that in order to use `org.springframework.security.data.repository.query.SecurityEvaluationContextExtension` you need `org.springframework.security` : `spring-security-data`

[joshlong]

Please explain in the docs that in order to use org.springframework.security.data.repository.query.SecurityEvaluationContextExtension you need org.springframework.security : spring-security-data. It's not clear from reading the section on accessing the java.security.Principal from SpEL that you need to add the spring-security-data library to the classpath.

[rwinch]

@joshlong Thanks for the suggestion. Would you like to send a PR?