Add documentation (#110)

docs/administration/oauth-providers.md

@@ -0,0 +1,39 @@
+# Use OAuth providers
+
+Opengist can be configured to use OAuth to authenticate users, with GitHub, Gitea, or OpenID Connect.
+
+## Github
+
+* Add a new OAuth app in your [Github account settings](https://github.com/settings/applications/new)
+* Set 'Authorization callback URL' to `http://opengist.domain/oauth/github/callback`
+* Copy the 'Client ID' and 'Client Secret' and add them to the [configuration](/docs/configuration/cheat-sheet.md) :
+  ```yaml
+  github.client-key: <key>
+  github.secret: <secret>
+  ```
+
+
+## Gitea
+
+* Add a new OAuth app in Application settings from the [Gitea instance](https://gitea.com/user/settings/applications)
+* Set 'Redirect URI' to `http://opengist.domain/oauth/gitea/callback`
+* Copy the 'Client ID' and 'Client Secret' and add them to the [configuration](/docs/configuration/cheat-sheet.md) :
+  ```yaml
+  gitea.client-key: <key>
+  gitea.secret: <secret>
+  # URL of the Gitea instance. Default: https://gitea.com/
+  gitea.url: http://localhost:3000
+  ```
+
+
+## OpenID Connect
+
+* Add a new OAuth app in Application settings of your OIDC provider
+* Set 'Redirect URI' to `http://opengist.domain/oauth/openid-connect/callback`
+* Copy the 'Client ID', 'Client Secret', and the discovery endpoint, and add them to the [configuration](/docs/configuration/cheat-sheet.md) :
+  ```yaml
+  oidc.client-key: <key>
+  oidc.secret: <secret>
+  # Discovery endpoint of the OpenID provider. Generally something like http://auth.example.com/.well-known/openid-configuration
+  oidc.discovery-url: http://auth.example.com/.well-known/openid-configuration
+  ```