OpenID - Discovery Endpoint - xrds mechanism

implements http://openid.net/specs/openid-authentication-2_0.html#discovery

Discovery is the process where the Relying Party uses the Identifier to look up ("discover") the necessary information for initiating requests. OpenID Authentication has three paths through which to do discovery:

If the identifier is an XRI, [XRI_Resolution_2.0] will yield an XRDS document that contains the necessary information. It should also be noted that Relying Parties can take advantage of XRI Proxy Resolvers, such as the one provided by XDI.org at http://www.xri.net. This will remove the need for the RPs to perform XRI Resolution locally.
    If it is a URL, the Yadis protocol [Yadis] SHALL be first attempted. If it succeeds, the result is again an XRDS document.
    If the Yadis protocol fails and no valid XRDS document is retrieved, or no Service Elements are found in the XRDS document, the URL is retrieved and HTML-Based discovery SHALL be attempted.