Init the organizations-service documentation

architecture/services/organizations.md

+# Organizations service
+
+
+
+## What it does
+This service provides a list of organizations with different information about it (such as description, logo..etc).  
+In our application, one organization is usually a provider of data, used in the Portail Open Data. It can be a public actor or a private one.
+
+## How it works
+
+![Diagram](/assets/organization-service.svg)
+
+The entrypoint of the service is a REST API provided by a [NestJS](https://github.com/nestjs/nest) application. This backend application interacts with a PostgresSQL database through an ORM called [TypeORM](https://github.com/typeorm/typeorm).
+
+
+## API documentation
+
+NestJS provides a swagger module that can be easily integrated. Using specific annotations alongside your endpoints declaration, this module will automatically generates a swagger documentation, reachable at `/api-docs`.
+
+## Service health
+
+NestJS provides a [health module](https://github.com/nestjs/terminus) based on Terminus, that gives you the opportunity to declare predifined or custom health indicators. It exposes the health status of the service at `/health`.  
+This service will return a `200` http status code when all indicators are healthy. Otherwise it will return a `503` http status code. 
+
+For this service we declared an health indicator that verifies that it can connect to the database.
+
+## Stats
+
+We are using a Node module called [swagger-stats](http://swaggerstats.io/).  
+It traces API calls, monitors API performance and usage statistics. It exposes the metrics in different formats, such as Prometheus format, so you may use Prometheus and Graphana for API monitoring and alterting.  
+Those metrics are available at `/swagger-stats/metrics`.
+
+For more information about this module, visit the [official swagger-stat page](http://swaggerstats.io/docs.html).
+
+## Docker
+It is possible to run this service using Docker containers, using the `docker-compose.yml` and `Dockerfile` files.  
+For more information, refer to the project [organizations-service][add a link]
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