Update README and remove usage of nested appenders in examples

[brenuart]

Declaring an <appender> inside another <appender> is not allowed anymore since Logback 1.3+ (see https://jira.qos.ch/browse/LOGBACK-1674 for more information).

We should update the documentation and remove sample configurations like the following:

<appender name="async" class="net.logstash.logback.appender.LoggingEventAsyncDisruptorAppender">
    <appender class="ch.qos.logback.core.rolling.RollingFileAppender">
        ...
    </appender>
</appender>

and replace them with:

<appender name="file" class="ch.qos.logback.core.rolling.RollingFileAppender">
    ...
</appender>

<appender name="async" class="net.logstash.logback.appender.LoggingEventAsyncDisruptorAppender">
    <appender-ref ref="file" />
</appender>