Customizing guide in readme (#41)

src/main/java/org/openmaptiles/addons/ExtraLayers.java

@@ -0,0 +1,19 @@
+package org.openmaptiles.addons;
+
+import com.onthegomap.planetiler.config.PlanetilerConfig;
+import com.onthegomap.planetiler.stats.Stats;
+import com.onthegomap.planetiler.util.Translations;
+import java.util.List;
+import org.openmaptiles.Layer;
+
+/**
+ * Registry of extra custom layers that you can add to the openmaptiles schema.
+ */
+public class ExtraLayers {
+
+  public static List<Layer> create(Translations translations, PlanetilerConfig config, Stats stats) {
+    return List.of(
+      // Create classes that extend Layer interface in the addons package, then instantiate them here
+    );
+  }
+}