Update custom.js: add default Pokémon to "Hide Pokémon". (#2471)

* Expanded 'custom.js' with another example.

A lot of people have been asking on how to exclude some Pokémon by default, so having it in the wiki is a good idea.

* Updated to Sebby's request.

* Tiny mistake fix

docs/extras/custom-js.md

@@ -13,7 +13,7 @@ Place your custom code into 'custom.js' in the folder 'static/js'. *Examples are
 Examples for a MOTD, google analytics and disabling scaling icons by rarity are included in custom.js.example!
 The example below is how to set default options usually done by editing core files.
 
-**Set a new map style by default.**
+* **Set a new map style by default.**  
 First we set a variable for it and pick which style we want as default.
 ```
 const map_style = 'satellite'
@@ -22,9 +22,31 @@ Next we have to tell the script to store the value to set it.
 ```
 Store.set('map_style', map_style)
 ```
+
 Whenever you edit custom.js you will have to run `npm run build` to set the changes.
 When you load the map it will be set to satellite as default.
 
-Setting options in this way forces that setting on page load, so even if a user changes the setting it will revert back to what you have set in custom.js every time, keep this in mind when forcing settings.
+Setting options in this way forces that setting on page load, so even if a user changes the setting it will revert back to what you have set in custom.js every time, keep this in mind when forcing settings.  
+
+If you don't want to revert back to what you have set in custom.js every time, you can do it like in the example below.  
+
+* **Add default Pokémon to "Hide Pokémon".**  
+First we need to retrieve what has already been set so we don't overwrite anything excluded in the interface. Then add the Pokémon we want to hide by default **by adding the Pokémon IDs to the excludedPokemon-array, e.g. [13, 16] as shown in the example below**. Finally we have to tell the script to store the value to set it.
+
+```
+const totalExcluded = Store.get('remember_select_exclude') // Retrieve what has already been set.
+
+const excludedPokemon = [13, 16] // Add Pokémon IDs separated by commas (e.g. [1, 2, 3])
+
+// If the new default Pokémon doesn't exist then add it to the list.
+excludedPokemon.forEach((id) => {
+    if (totalExcluded.indexOf(id) === -1) {
+        totalExcluded.push(id)
+    }
+})
+
+Store.set('remember_select_exclude', totalExcluded) // Tell the script to store the value to set it.
+``` 
+When you load the map Weedle and Pidgey will be hidden as default.
 
 See 'custom.js.example' for more information.