Add Gitea connector (#1715)

* Add Gitea connector

* Add details to readme

* resolve lint issue

Documentation/connectors/gitea.md

@@ -0,0 +1,29 @@
+# Authentication through Gitea
+
+## Overview
+
+One of the login options for dex uses the Gitea OAuth2 flow to identify the end user through their Gitea account.
+
+When a client redeems a refresh token through dex, dex will re-query Gitea to update user information in the ID Token. To do this, __dex stores a readonly Gitea access token in its backing datastore.__ Users that reject dex's access through Gitea will also revoke all dex clients which authenticated them through Gitea.
+
+## Configuration
+
+Register a new OAuth consumer with [Gitea](https://docs.gitea.io/en-us/oauth2-provider/) ensuring the callback URL is `(dex issuer)/callback`. For example if dex is listening at the non-root path `https://auth.example.com/dex` the callback would be `https://auth.example.com/dex/callback`.
+
+The following is an example of a configuration for `examples/config-dev.yaml`:
+
+```yaml
+connectors:
+- type: gitea
+  # Required field for connector id.
+  id: gitea
+  # Required field for connector name.
+  name: Gitea
+  config:
+    # Credentials can be string literals or pulled from the environment.
+    clientID: $GITEA_CLIENT_ID
+    clientSecret: $GITEA_CLIENT_SECRET
+    redirectURI: http://127.0.0.1:5556/dex/callback
+    # optional, default = https://gitea.com
+    baseURL: https://gitea.com
+```