Add a page on installing plugins

This closes #2491.

tutorials/plugins/editor/index.rst

@@ -5,6 +5,7 @@ Editor plugins
    :maxdepth: 1
    :name: toc-tutorials-plugins
 
+   installing_plugins
    making_plugins
    making_main_screen_plugins
    import_plugins

tutorials/plugins/editor/installing_plugins.rst

@@ -0,0 +1,70 @@
+.. _doc_installing_plugins:
+
+Installing plugins
+==================
+
+Godot features an editor plugin system with numerous plugins developed by the
+community. Plugins can extend the editor's functionality with new nodes,
+additional docks, convenience features, and more.
+
+Finding plugins
+~~~~~~~~~~~~~~~
+
+The preferred way to find Godot plugins is to use the
+`Asset Library <https://godotengine.org/asset-library/>`_. While it can be
+browsed online, it's more convenient to use it directly from the editor.
+To do so, click the **AssetLib** tab at the top of the editor:
+
+.. image:: img/installing_plugins_assetlib_tab.png
+
+You can also find assets on code hosting websites such as GitHub.
+
+.. note::
+
+    Some repositories describe themselves as "plugins" but may not actually be
+    *editor* plugins. This is especially the case for scripts that are intended
+    to be used in a running project. You don't need to enable such plugins to
+    use them. Download them and extract the files in your project folder.
+
+    One way to distinguish editor plugins from non-editor plugins is to look for
+    a ``plugin.cfg`` file in the repository that hosts the plugin. If the
+    repository contains a ``plugin.cfg`` file in a folder placed in the
+    ``addons/`` folder, then it is an editor plugin.
+
+Installing a plugin
+~~~~~~~~~~~~~~~~~~~
+
+To install a plugin, download it as a ZIP archive. On the Asset Library, this
+can be done using the **Download** button, either from the editor or using the
+Web interface.
+
+On GitHub, if a plugin has *tags* (versions) declared, go to the **Releases**
+tab to download a stable release. This ensures you download a version that was
+declared to be stable by its author.
+
+On GitHub, if the plugin doesn't have any *tags* declared, use the **Download ZIP**
+button to download a ZIP of the latest revision:
+
+.. image:: img/installing_plugins_github_download_zip.png
+
+Extract the ZIP archive and move the ``addons/`` folder it contains into your
+project folder. If your project already contains an ``addons/`` folder, move the
+plugin's ``addons/`` folder into your project folder to merge the new folder
+contents with the existing one. Your file manager may ask you whether to write
+into the folder; answer **Yes**. No files will be overwritten in the process.
+
+Enabling a plugin
+~~~~~~~~~~~~~~~~~
+
+To enable the freshly installed plugin, open **Project > Project Settings** at
+the top of the editor then go the **Plugins** tab. If the plugin was packaged
+correctly, you should see it in the list of plugins. Click on the gray
+**Inactive** text and choose **Active** to enable the plugin. The word
+**Active** will display in green to confirm the plugin was enabled.
+
+.. image:: img/installing_plugins_project_settings.png
+
+
+You can use the plugin immediately after enabling it; there's no need to restart
+the editor. Likewise, disabling a plugin can be done without having to restart
+the editor.