Add flags (#21)

Author: mbilski

README.md

@@ -36,7 +36,30 @@ To use `oauth2c`, run the following command and follow the prompts:
 oauth2c [issuer url] [flags]
 ```
 
-You will be asked to provide the necessary information, such as the grant type, client authentication method, and any other relevant details.
+The available flags are:
+
+``` sh
+      --assertion string         claims for jwt bearer assertion
+      --auth-method string       token endpoint authentication method
+      --client-id string         client identifier
+      --client-secret string     client secret
+      --grant-type string        grant type
+      --insecure                 allow insecure connections
+      --no-pkce                  disable proof key for code exchange (PKCE)
+      --password string          resource owner password credentials grant flow password
+      --pkce                     enable proof key for code exchange (PKCE)
+      --refresh-token string     refresh token
+      --response-mode string     response mode
+      --response-types strings   response type
+      --scopes strings           requested scopes
+      --signing-key string       path or url to signing key in jwks format
+      --tls-cert string          path to tls cert pem file
+      --tls-key string           path to tls key pem file
+      --tls-root-ca string       path to tls root ca pem file
+      --username string          resource owner password credentials grant flow username
+```
+
+You will be asked to provide the necessary information, such as the grant type, client authentication method, and any other relevant details (if not already provided).
 
 Once authenticated, you will be able to use the access token to access the OAuth2 provider's API.