docs(faq): added loglevelfilter example to faq

docs/faq.md

@@ -0,0 +1,23 @@
+# Frequently Asked Questions
+
+## I want errors to go to a special file, but still want everything written to another file - how do I do that?
+
+You'll need to use the [logLevelFilter](logLevelFilter.md). Here's an example configuration:
+```javascript
+log4js.configure({
+  appenders: {
+    everything: { type: 'file', filename: 'all-the-logs.log' },
+    emergencies: {  type: 'file', filename: 'oh-no-not-again.log' },
+    'just-errors': { type: 'logLevelFilter', appender: 'emergencies', minLevel: 'error' }
+  },
+  categories: {
+    default: { appenders: ['just-errors', 'everything'], level: 'debug' }
+  }
+});
+
+const logger = log4js.getLogger();
+logger.debug('This goes to all-the-logs.log');
+logger.info('As does this.');
+logger.error('This goes to all-the-logs.log and oh-no-not-again.log');
+
+```