Add docs about using env vars for credentials with omegaconfloader

docs/source/kedro_project_setup/configuration.md

@@ -273,6 +273,19 @@ data:
 Since both of the file names (`parameters.yml` and `parameters_globals.yml`) match the config pattern for parameters, the `OmegaConfLoader` will load the files and resolve the placeholders correctly.
 
 
+### Environment variables for credentials
+The [`OmegaConfLoader`](/kedro.config.OmegaConfLoader) enables you to load credentials from environment variables. To achieve this you have to use the `omegaconf` [`oc.env` resolver](https://omegaconf.readthedocs.io/en/2.3_branch/custom_resolvers.html#oc-env). 
+This is an example of how you can access credentials from environment variables in `credentials.yml`:
+
+```yaml
+dev_s3:
+  client_kwargs:
+    aws_access_key_id: ${oc.env:AWS_ACCESS_KEY_ID}
+    aws_secret_access_key: ${oc.env:AWS_SECRET_ACCESS_KEY}
+```
+
+Note that you can only use the resolver in `credentials.yml` and not in catalog or parameter files. This is because we do not encourage the usage of environment variables for anything other than credentials. 
+
 ## Parameters
 
 ### Load parameters