Moved extensions docs draft from wiki to main repository

docs/guide/extensions.md

 Extending Yii
 =============
 
-TDB
+Code style
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+----------
+
+- 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.
+
+Distribution
+------------
+
+- There should be a `readme.md` file clearly describing what extension does in English, its requirements, how to install
+  and use it. It should be written using markdown. If you want to provide translated readme, name it as `readme_ru.md`
+  where `ru` is your language code.
+- TBD: composer.json
+
+Working with database
+---------------------
+
+- If extension creates or modifies database schema always use Yii migrations instead of SQL files or custom scripts.
+
+Assets
+------
+
+TBD
+
+Events
+------
+
+TBD
+
+i18n
+----
+
+TBD
+
+Testing your extension
+----------------------
+
+TBD
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