Document plugin settings

Ported from stashapp/stash@2e00cb6

in-app-manual/plugins.md

@@ -115,6 +115,16 @@ ui:
     connect-src:
       - http://alloweddomain.com
 
+  # map of setting names to be displayed in the plugins page in the UI
+  settings:
+    # internal name
+    foo:
+      # name to display in the UI
+      displayName: Foo
+      # type of the attribute to show in the UI
+      # can be BOOLEAN, NUMBER, or STRING
+      type: BOOLEAN
+
 # the following are used for plugin tasks only
 exec:
   - ...
@@ -128,6 +138,8 @@ The `name`, `description`, `version` and `url` fields are displayed on the plugi
 
 The `exec`, `interface`, `errLog` and `tasks` fields are used only for plugins with tasks.
 
+The `settings` field is used to display plugin settings on the plugins page. Plugin settings can also be set using the graphql mutation `configurePlugin` - the settings set this way do _not_ need to be specified in the `settings` field unless they are to be displayed in the stock plugin settings UI.
+
 ## UI Configuration
 
 The `css` and `javascript` field values may be relative paths to the plugin configuration file, or