Add customization introduction sections

docs/en/customization/introduction.md

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+# Customization
+
+You can modify your own CONSUL to have your custom visual style, but first you'll have to [create your own fork from](forks/create.md).
+
+We've created an specific structure where you can overwrite and customize the application in a way that will let you keep updating it from CONSUL's main repository, without having conflicts on code merging or risking loosing your customization changes. We try to make CONSUL as vanilla as possible to help other developers onboard the codebase.
+
+## Special Folders and Files
+
+In order to customize your CONSUL fork, you'll make use of some `custom` folders on the following paths:
+
+* `config/locales/custom/`
+* `app/assets/images/custom/`
+* `app/views/custom/`
+* `app/controllers/custom/`
+* `app/models/custom/`
+
+Also these are the files where you can apply some customization:
+
+* `app/assets/stylesheets/custom.css`
+* `app/assets/stylesheets/_custom_settings.css`
+* `app/assets/javascripts/custom.js`
+* `Gemfile_custom`
+* `config/application.custom.rb`