Add ProGuard rules to the FAQ (#2182)

Author: Goooler

src/site/markdown/faq.md.vm

@@ -37,6 +37,7 @@ $h1 Frequently Asked Questions
 * [Is Log4j 2 still garbage-free when I use the SLF4J API?](#gc-free-slf4j)
 * [How do I log my domain object without creating garbage?](#gc-free-domain-object)
 * [How do I create a custom logger wrapper that shows the correct class, method and line number?](#logger-wrapper)
+* [Which rules do I need to add when minification is enabled?](#proguard-rules)
 
 <a name="missing_core"/>
 
@@ -494,3 +495,12 @@ This tool was originally meant to support custom log levels and is documented
 [here](https://logging.apache.org/log4j/2.x/manual/customloglevels.html#CustomLoggers).
 
 The generated logger code will take care of the FQCN.
+
+<a name="proguard-rules" />
+
+$h4 Which rules do I need to add when ProGuard minification is enabled?
+
+When you are using Log4j with ProGuard/R8 enabled, you need to add the following rules to your configuration file:
+```
+-keep,allowoptimization class org.apache.logging.log4j.** { *; }
+```