update 'interoperable SAML' link in readme

[andrascz]

Important notice
The work on saml2int has moved to Kantara Initiative. This web site refers to older versions of saml2int. If your identity provider or federation links to this page, please ask them to update the links to the latest versino of the deployment profile. Here is the link to the latest version of Kantara SAML V2.0 Interoperability Deployment Profile:
kantarainitiative.github.io/SAMLprofiles/saml2int.html

via https://saml2int.org/