add basic readme

Signed-off-by: Frank Villaro-Dixon <frank@villaro-dixon.eu>

README.md

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+# Rust RTE France
+
+This is an api around the French TSO [RTE](https://rte-france.com).
+
+The main endpoints [live here](https://data.rte-france.com).
+
+
+## Provided APIs
+
+
+### Generation forecast API
+
+### Consumption forecast API
+
+## Authentication
+
+To use the API, you need to register an account in RTE's system (free).
+
+Then you need to create an application. To do that,
+
+- Go to [this application](https://data.rte-france.com/catalog/-/api/consumption/Consumption/v1.2)
+- Click on "Abonnez vous à l'API"
+- Fill the "Créer une application" form
+- Then go to you application [here](https://data.rte-france.com/group/guest/apps)
+- And copy the Client ID and Client Secret
+
+These two variables need to be set as environment values:
+
+- `CLIENT_ID`
+- `CLIENT_SECRET`
+