plugin/auth-backend: update docs

plugins/auth-backend/README.md

@@ -7,17 +7,46 @@ This is the backend part of the auth plugin.
 It responds to auth requests from the frontend, and fulfills them by delegating
 to the appropriate provider in the backend.
 
-## Requirements
-
-Needs AUTH_GOOGLE_CLIENT_ID and AUTH_GOOGLE_CLIENT_SECRET set in the environment for the backend to startup
-
 ## Local development
 
-export AUTH_GOOGLE_CLIENT_ID=<INSERT_CLIENT_ID_HERE>
-read -r AUTH_GOOGLE_CLIENT_SECRET
-<COPY_PASTE_CLIENT_SECRET_HERE>
-export AUTH_GOOGLE_CLIENT_SECRET
-run `yarn start` in packages/backend folder
+Choose your OAuth Providers, replace `x` with actual value and then start backend:
+Example for Google Oauth Provider at root directory:
+
+```bash
+export AUTH_GOOGLE_CLIENT_ID=x
+export AUTH_GOOGLE_CLIENT_SECRET=x
+yarn --cwd packages/backend start
+```
+
+### Google
+
+```bash
+export AUTH_GOOGLE_CLIENT_ID=x
+export AUTH_GOOGLE_CLIENT_SECRET=x
+```
+
+### Github
+
+```bash
+export AUTH_GITHUB_CLIENT_ID=x
+export AUTH_GITHUB_CLIENT_SECRET=x
+```
+
+### Gitlab
+
+```bash
+export GITLAB_BASE_URL=x # default is https://gitlab.com
+export AUTH_GITLAB_CLIENT_ID=x
+export AUTH_GITLAB_CLIENT_SECRET=x
+```
+
+### Okta
+
+```bash
+export AUTH_OKTA_AUDIENCE=x
+export AUTH_OKTA_CLIENT_ID=x
+export AUTH_OKTA_CLIENT_SECRET=x
+```
 
 ### SAML