document the application customization

Describe how to customize the following thins:

- [x] templates
- [x] translations
- [x] forms
- [x] validation
- [x] controllers
- [x] services
- [x] create custom entities
- [x] UI bundle placeholders
- [x] OroTranslationBundle
- [x] adding field to an existing entity
- [x] adding regular entity and migration script
It would be good to write about bundles priority (here or in the separate chapter). Example is here: https://github.com/orocrm/platform/blob/master/src/Oro/Bundle/SecurityBundle/Resources/config/oro/bundles.yml.