Update docs to prevent null pointer exception when instrumenting log4j

Ran into null pointer exceptions using v3.0.1 when using slf4j 1.6.4 and log4j 1.2.16 due to activateOptions not having been called, causing counters to be instantiated, before attempting to log things.

```
java.lang.ExceptionInInitializerError: null
	at com.codahale.metrics.log4j.InstrumentedAppender.append(InstrumentedAppender.java:61)
	at org.apache.log4j.AppenderSkeleton.doAppend(AppenderSkeleton.java:251)
	at org.apache.log4j.helpers.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:66)
	at org.apache.log4j.Category.callAppenders(Category.java:206)
	at org.apache.log4j.Category.forcedLog(Category.java:391)
	at org.apache.log4j.Category.log(Category.java:856)
	at org.slf4j.impl.Log4jLoggerAdapter.info(Log4jLoggerAdapter.java:305)
```

Failing example:
```fail.java
        LogManager.getRootLogger().addAppender(new InstrumentedAppender(registry));
        LoggerFactory.getLogger(Metrics.class).info("Test");
```

Passing example:
```pass.java
        InstrumentedAppender appender = new InstrumentedAppender(registry);
        appender.activateOptions();
        LogManager.getRootLogger().addAppender(appender);
        LoggerFactory.getLogger(Metrics.class).info("Test");
```