Update README

README.md

@@ -5,10 +5,12 @@ Basic & lightweight OpenID provider, written in Rust using the Actix framework.
 
 **WARNING :** This tool has not been audited, use it at your own risks!
 
-BasicOIDC operates without any database, just with two files :
+BasicOIDC operates without any database, just with three files :
 * `clients.yaml`: a list of authorized relying parties.
+* `providers.yaml`: a list of upstream providers for authentication federation (this file is optional)
 * `users.json`: a list of users, managed through a web UI.
 
+## Configuration
 You can configure a list of clients (Relying Parties) in a `clients.yaml` file with the following syntax :
 ```yaml
 - id: gitea
@@ -26,7 +28,7 @@ On the first run, BasicOIDC will create a new administrator with credentials `ad
 
 In order to run BasicOIDC for development, you will need to create a least an empty `clients.yaml` file inside the storage directory.
 
-Features :
+## Features
 * [x] `authorization_code` flow
 * [x] Client authentication using secrets 
 * [x] Bruteforce protection 
@@ -35,6 +37,21 @@ Features :
   * [x] Using a security key (Webauthn)
 * [ ] Fully responsive webui
 * [x] `robots.txt` prevents indexing
+* [x] Support authentication from upstream provider
+
+## Add an upstream provider
+You can add as much upstream provider as you want, using the following syntax in `providers.yaml`:
+```yaml
+- id: gitlab
+  name: GitLab
+  logo: gitlab # Can be either gitea, gitlab, github, microsoft, google or a full URL
+  client_id: CLIENT_ID_GIVEN_BY_PROVIDER
+  client_secret: CLIENT_SECRET_GIVEN_BY_PROVIDER
+  configuration_url: https://gitlab.com/.well-known/openid-configuration
+
+```
+
+> Warning! Self-registration has not been implemented, therfore the accounts must have been previously created through the administration.
 
 ## Compiling
 You will need the Rust toolchain to compile this project. To build it for production, just run: