Added documentation for gateway configuration files

gateway/README.md

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+This folder contains configuration files and custom templates for geOrchestra Gateway.
+
+## Documentation
+
+Documentation can be found in [docs directory at root of georchestra-gateway](https://github.com/georchestra/georchestra-gateway/blob/ui_customization_doc/docs) repository.
+
+## List of configuration files
+
+#### `application.yaml`
+
+Contains filters applicable to all transfers through the gateway, and general Spring settings.
+
+#### `gateway.conf`
+
+Contains access rules for all services available through the gateway.
+
+Please see https://github.com/georchestra/georchestra-gateway/blob/main/docs/access-rules.adoc
+
+#### `roles-mappings.yaml`
+
+Allow to map roles returned by authentication providers to standardized geOrchestra roles.
+
+Please see https://github.com/georchestra/georchestra-gateway/blob/main/docs/roles-mappings.adoc
+
+####  `routes.yaml`
+
+Contains list of routes for redirection to the correct service URL based on criterias (typically URL patterns but other criterias exists).  
+Also allows to apply additional filters on this route.
+
+Please see https://cloud.spring.io/spring-cloud-gateway/reference/html
+
+####  `security.yaml`
+
+Contains all settings about authentication. Please see the following documents :  
+OAuth2 : https://github.com/georchestra/georchestra-gateway/blob/main/docs/authzn.adoc  
+Pre-authentication : https://github.com/georchestra/georchestra-gateway/blob/main/docs/pre-authentication.adoc  
+
+## Templates
+
+If needed, default pages can be overriden by custom templates. This includes login, logout page, and error pages. Only existing templates will override default ones.
+
+Please see https://github.com/georchestra/georchestra-gateway/blob/main/docs/ui-customization.adoc