Docs: mention that core plugins can be whitelisted in two places

docs/source/plugin_development.rst

@@ -115,6 +115,10 @@ If you're adding a "core" plugin, which means a plugin that will be
 included in the default, open-source version of converse.js, then you'll
 instead whitelist the plugin by adding its name to the `core_plugins` array in
 `./src/headless/converse-core.js <https://github.com/jcbrand/converse.js/blob/master/src/headless/converse-core.js>`_.
+or the `WHITELISTED_PLUGINS` array in `./src/converse.js <https://github.com/jcbrand/converse.js/blob/master/src/converse.js>`_.
+
+Where you add it depends on whether your plugin is part of the headless build
+(which means it doesn't contain any view code) or not.
 
 Security and access to the inner workings
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