related to #1693 changed smarty version to 4.1.0

include/smarty/docs/programmers/plugins/plugins-howto.md

@@ -0,0 +1,18 @@
+How Plugins Work {#plugins.howto}
+================
+
+Plugins are always loaded on demand. Only the specific modifiers,
+functions, resources, etc invoked in the templates scripts will be
+loaded. Moreover, each plugin is loaded only once, even if you have
+several different instances of Smarty running within the same request.
+
+Pre/postfilters and output filters are a bit of a special case. Since
+they are not mentioned in the templates, they must be registered or
+loaded explicitly via API functions before the template is processed.
+The order in which multiple filters of the same type are executed
+depends on the order in which they are registered or loaded.
+
+The [plugins directory](#variable.plugins.dir) can be a string
+containing a path or an array containing multiple paths. To install a
+plugin, simply place it in one of the directories and Smarty will use it
+automatically.