`Logging Sensitive Data` section is outdated in README.md

[grimsa]

https://github.com/AuthorizeNet/sdk-java#logging-sensitive-data seems to be outdated after the switch to Log4j 2.
Link to log4j.properties is broken as well.

Ideally, the facilities for masking sensitive data should:

• either be provided for all popular logging frameworks (with provided dependency scope, in order to not make them transitive dependencies, see SDK should not pull non-API logging dependencies #170)
• or be generic, making it easy to wrap them to adapt to the framework the project consuming the SDK is using