Added docs for i81n.moduole_priority.

docs/en/topics/i18n.md

@@ -223,6 +223,40 @@ If you want to run the text collector for just one module you can use the 'modul
 You'll need to install PHPUnit to run the text collector (see [testing-guide](/topics/testing)).
 </div>
 
+## Module Priority
+
+The order in which i18n strings are loaded from modules can be quite important, as it is pretty common for a site
+developer to want to override the default i18n strings from time to time.  Because of this, you will sometimes need to specify the loading priority of i18n modules.
+
+By default, the language files are loaded from modules in this order:
+
+ * Your project (as defined in the `$project` global)
+ * admin
+ * framework
+ * All other modules
+
+This default order is configured in `framework/_config/i18n.yml`.  This file specifies two blocks of module ordering: `basei18n`, listing admin, and framework, and `defaulti18n` listing all other modules.
+
+To create a custom module order, you need to specify a config fragment that inserts itself either after or before those items.  For example, you may have a number of modules that have to come after the framework/admin, but before anyhting else.  To do that, you would use this 
+
+	---
+	Name: customi18n
+	Before: 'defaulti18n'
+	---
+	i18n:
+	  module_priority:
+	    - module1
+	    - module2
+	    - module3
+
+The config option being set is `i18n.module_priority`, and it is a list of module names.
+
+There are a few special cases:
+
+ * If not explicitly mentioned, your project is put as the first module.
+ * The module name `other_modules` can be used as a placeholder for all modules that aren't
+   specifically mentioned.
+
 ## Language definitions
 
 Each module can have one language table per locale, stored by convention in the `lang/` subfolder.