Migrate to SLF4J (#157)

src/main/java/net/fabricmc/example/ExampleMod.java

 package net.fabricmc.example;
 
 import net.fabricmc.api.ModInitializer;
-import org.apache.logging.log4j.LogManager;
-import org.apache.logging.log4j.Logger;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 public class ExampleMod implements ModInitializer {
 	// This logger is used to write text to the console and the log file.
 	// It is considered best practice to use your mod id as the logger's name.
 	// That way, it's clear which mod wrote info, warnings, and errors.
-	public static final Logger LOGGER = LogManager.getLogger("modid");
+	public static final Logger LOGGER = LoggerFactory.getLogger("modid");
 
 	@Override
 	public void onInitialize() {