doc: Documenting how to configure s3 backend with env vars (#604)

Author: jeansebastienh

docs/index.md

@@ -182,3 +182,10 @@ terraform {
 
 Beware as no locking mechanism are yet supported.
 Using scaleway object storage as terraform backend is not suitable if you work in a team with a risk of simultaneous access to the same plan.
+
+Note: For security reason it's not recommended to store secrets in terraform files. If you want to configure the backend with environnment var, you need to use `AWS_ACCESS_KEY_ID` and `AWS_SECRET_ACCESS_KEY` [source](https://www.terraform.io/docs/backends/types/s3.html#access_key).
+
+```bash
+export AWS_ACCESS_KEY_ID=$SCW_ACCESS_KEY
+export AWS_SECRET_ACCESS_KEY=$SCW_SECRET_KEY
+```