Mention plugin format (#31)

Explicitly mention that you have to unzip plugins.

Signed-off-by: Volker Schlegel <volker.schlegel@tutanota.com>

docs/custom-content.md

@@ -71,6 +71,7 @@ Peacock supports a broad variety of server side mods (plugins). These include, b
 
 -   Broadly speaking, to install plugins, go to either the `plugins` or `#plugin-releases` channel in the Peacock Discord and download a plugin you are interested in.
 -   -   Put your plugins into the `plugins` folder in the Peacock folder.
+    -   Your plugin should end with .plugin.js - if its distributed as a .zip extract it first.
     -   Example: ![PluginsFolder](/img/plugins_folder.png)
 -   Be sure to check for any further instructions in the `#plugin-releases` channel for a given plugin, as certain plugins may require further installation steps or are incompatible with newer versions of Peacock.