Added persist explanation for internal expansions provided by the dependency

PlaceholderExpansion.md

@@ -292,6 +292,8 @@ class SomeExpansion extends PlaceholderExpansion {
 #### Internal class
 You can have the class inside your plugin.  
 This has some advantages but can also have some disadvantages.
+If you include a PlaceholderExpansion class in your plugin, you MUST add and override the persist() method to return true
+otherwise if PlaceholderAPI is reloaded, your expansion will be unregistered and lost forever.
 
 
 ```java
@@ -321,6 +323,18 @@ class SomeExpansion extends PlaceholderExpansion {
         this.plugin = plugin;
     }
 
+    /**
+     * Because this is an internal class,
+     * you must override this method to let PlaceholderAPI know to not unregister your expansion class when
+     * PlaceholderAPI is reloaded
+     *
+     * @return true to persist through reloads
+     */
+    @Override
+    public boolean persist(){
+        return true;
+    }
+
     /**
      * Because this is a internal class, this check is not needed
      * and we can simply return {@code true}