Add authentication method for the token endpoint

Author: Roman Minkin

README.md

@@ -511,6 +511,7 @@ request_url | [oauth.json][oauth-config] | OAuth1/step1
 authorize_url | [oauth.json][oauth-config] | OAuth1/step2 or OAuth2/step1
 access_url | [oauth.json][oauth-config] | OAuth1/step3 or OAuth2/step2
 oauth | [oauth.json][oauth-config] | OAuth version number
+token_endpoint_auth_method | [oauth.json][oauth-config] | Authentication method for the token endpoint, default `client_secret_basic`
 scope_delimiter | [oauth.json][oauth-config] | string delimiter used for concatenating multiple scopes
 custom_parameters | [oauth.json][oauth-config] | list of known custom authorization parameters
 protocol, host, path | `defaults` | used to generate `redirect_uri`