Add comment about token replacement

src/main/java/com/myname/mymodid/Tags.java

@@ -2,6 +2,7 @@ package com.myname.mymodid;
 
 public class Tags {
 
+    // GRADLETOKEN_* will be replaced by your configuration values at build time
     public static final String MODID = "GRADLETOKEN_MODID";
     public static final String MODNAME = "GRADLETOKEN_MODNAME";
     public static final String VERSION = "GRADLETOKEN_VERSION";