docs: update readme

Signed-off-by: Rintaro Okamura <[email protected]>

README.md

@@ -1,5 +1,8 @@
 # Google Cloud Auth Proxy
 
+[![latest tag](https://ghcr-badge.egpl.dev/rinx/gcloud-auth-proxy/latest_tag?trim=major&label=latest)](https://github.com/users/rinx/packages/container/package/gcloud-auth-proxy)
+[![image size](https://ghcr-badge.egpl.dev/rinx/gcloud-auth-proxy/size)](https://github.com/users/rinx/packages/container/package/gcloud-auth-proxy)
+
 This is a software that provides [Google-signed authentication tokens][google-token] in several ways.
 
 Currently, it supports the following token types.
@@ -9,11 +12,32 @@ Currently, it supports the following token types.
 [google-token]: https://cloud.google.com/docs/authentication/token-types
 [google-id-token]: https://cloud.google.com/docs/authentication/get-id-token#go
 
-## Usage
+## Usecase
 
 TBW
 
-## Endpoints
+## Usage
+
+Deploy gcloud-auth-proxy as a sidecar with your application.
+
+Container is available on `ghcr.io/rinx/gcloud-auth-proxy:latest`.
+
+CLI flags are the followings.
+
+```bash
+Usage:
+  gcloud-auth-proxy [flags]
+
+Flags:
+      --audience string                      default audience (required)
+  -h, --help                                 help for gcloud-auth-proxy
+      --host string                          server host (default "0.0.0.0")
+      --port string                          server port (default "8100")
+      --token-source-cache-duration string   token source cache duration (default "30m")
+  -v, --version                              version for gcloud-auth-proxy
+```
+
+### Endpoints
 
 | endpoint         | method | description      |
 |------------------|--------|------------------|

pkg/service/google/idtoken/idtoken.go

@@ -118,8 +118,6 @@ func (s *server) idTokenHandler(w http.ResponseWriter, r *http.Request) {
 
 	w.Header().Set("Content-Type", "application/json")
 	w.Header().Set("Cache-Control", "no-store")
-
-	w.WriteHeader(http.StatusOK)
 }
 
 func (s *server) idTokenProxy(w http.ResponseWriter, r *http.Request) {