chore: [README] Added "OAuth 2.0 Form Post Response Mode"

Author: Giuseppe De Marco

README.md

@@ -21,6 +21,7 @@ As OIDC OP Idpy implements the following standards:
 * [OpenID Connect Back-Channel Logout 1.0](https://openid.net/specs/openid-connect-backchannel-1_0.html)
 * [OpenID Connect Front-Channel Logout 1.0](https://openid.net/specs/openid-connect-frontchannel-1_0.html)
 * [OAuth2 Token introspection](https://tools.ietf.org/html/rfc7662)
+* [OAuth 2.0 Form Post Response Mode](https://openid.net/specs/oauth-v2-form-post-response-mode-1_0.html)
 
 It also comes with the following `add_on` modules.