Added rules about namespace and package naming to extension docs

docs/guide/extensions.md

@@ -6,12 +6,19 @@ Code style
 
 - Extension code style should be similar to [core framework code style](https://github.com/yiisoft/yii2/wiki/Core-framework-code-style).
 - In case of using getter and setter for defining a property it's preferred to use method in extension code rather than property.
-- TBD: namespace
 - All classes, methods and properties should be documented using phpdoc. Note that you can use markdown and like to API
 documents using `[[name()]]`.
 - If you're displaying errors to developers do not translate these (i.e. do not use `\Yii::t()`). Errors should be
   translated only if they're displayed to end users.
 
+### Namespace and package names
+
+- Extension MUST use the type `yii2-extension` in `composer.json` file.
+- Extension MUST NOT use `yii` or `yii2` in the composer package name or in the namespaces used in the package.
+- Extension SHOULD use namespaces in this format `vendor-name\package` (all lowercase).
+- Extension MAY use a `yii2-` prefix in the composer vendor name (URL).
+- Extension MAY use a `yii2-` prefix in the repository name (URL).
+
 Distribution
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