add references to client authentication method names from RFC7591

closes #210

Author: aaronpk

draft-ietf-oauth-v2-1.md

@@ -1072,6 +1072,8 @@ parameters:
 The parameters can only be transmitted in the request content and MUST NOT
 be included in the request URI.
 
+This is also known as `client_secret_post` as defined in {{Section 2 of RFC7591}}.
+
 For example, a request to refresh an access token ({{refreshing-an-access-token}}) using
 the content parameters (with extra line breaks for display purposes
 only):
@@ -1094,6 +1096,8 @@ to authenticate with the authorization server, the client identifier is encoded
 secret is encoded using the same algorithm and used as the
 password.
 
+This is also known as `client_secret_basic` as defined in {{Section 2 of RFC7591}}.
+
 For example (with extra line breaks for display purposes only):
 
     Authorization: Basic czZCaGRSa3F0Mzo3RmpmcDBaQnIxS3REUmJuZlZkbUl3