Add sp credentials to docs

docs/website/docs/dlt-ecosystem/destinations/filesystem.md

@@ -150,7 +150,13 @@ Use **Cloud Storage** admin to create a new bucket. Then assign the **Storage Ob
 #### Azure Blob Storage
 Run `pip install dlt[az]` which will install the `adlfs` package to interface with Azure Blob Storage.
 
-Edit the credentials in `.dlt/secrets.toml`, you'll see AWS credentials by default replace them with your Azure credentials:
+Edit the credentials in `.dlt/secrets.toml`, you'll see AWS credentials by default replace them with your Azure credentials.
+
+Two forms of Azure credentials are supported:
+
+##### SAS token credentials
+
+Supply storage account name and either sas token or storage account key
 
 ```toml
 [destination.filesystem]
@@ -168,6 +174,20 @@ If you have the correct Azure credentials set up on your machine (e.g. via azure
 you can omit both `azure_storage_account_key` and `azure_storage_sas_token` and `dlt` will fall back to the available default.
 Note that `azure_storage_account_name` is still required as it can't be inferred from the environment.
 
+##### Service principal credentials
+
+Supply a client ID, client secret and a tenant ID for a service principal authorized to access your container
+
+```toml
+[destination.filesystem]
+bucket_url = "az://[your_container name]" # replace with your container name
+
+[destination.filesystem.credentials]
+azure_client_id = "client_id" # please set me up!
+azure_client_secret = "client_secret
+azure_tenant_id = "tenant_id" # please set me up!
+```
+
 #### Local file system
 If for any reason you want to have those files in a local folder, set up the `bucket_url` as follows (you are free to use `config.toml` for that as there are no secrets required)
 
@@ -458,4 +478,4 @@ managed in the regular way by the final destination you have configured.
 You will also notice `init` files being present in the root folder and the special `dlt` folders. In the absence of the concepts of schemas and tables
 in blob storages and directories, `dlt` uses these special files to harmonize the behavior of the `filesystem` destination with the other implemented destinations.
 
-<!--@@@DLT_TUBA filesystem-->
+<!--@@@DLT_TUBA filesystem-->