Add Gitea to auth config docs (#510)

* add gitea to auth config docs

* PR feedback

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

docs/2_auth.md

@@ -24,6 +24,7 @@ Valid providers are :
 - [Nextcloud](#nextcloud-provider)
 - [DigitalOcean](#digitalocean-auth-provider)
 - [Bitbucket](#bitbucket-auth-provider)
+- [Gitea](#gitea-auth-provider)
 
 The provider can be selected using the `provider` configuration value.
 
@@ -397,6 +398,25 @@ To use the provider, pass the following options:
 The default configuration allows everyone with Bitbucket account to authenticate. To restrict the access to the team members use additional configuration option: `--bitbucket-team=<Team name>`. To restrict the access to only these users who has access to one selected repository use `--bitbucket-repository=<Repository name>`.
 
 
+### Gitea Auth Provider
+
+1. Create a new application: `https://< your gitea host >/user/settings/applications`
+2. Under `Redirect URI` enter the correct URL i.e. `https://<proxied host>/oauth2/callback`
+3. Note the Client ID and Client Secret.
+4. Pass the following options to the proxy:
+
+```
+    --provider="github"
+    --redirect-url="https://<proxied host>/oauth2/callback"
+    --provider-display-name="Gitea"
+    --client-id="< client_id as generated by Gitea >"
+    --client-secret="< client_secret as generated by Gitea >"
+    --login-url="https://< your gitea host >/login/oauth/authorize"
+    --redeem-url="https://< your gitea host >/login/oauth/access_token"
+    --validate-url="https://< your gitea host >/api/v1"
+```
+
+
 ## Email Authentication
 
 To authorize by email domain use `--email-domain=yourcompany.com`. To authorize individual email addresses use `--authenticated-emails-file=/path/to/file` with one email per line. To authorize all email addresses use `--email-domain=*`.