[Openid-specs-fapi] Issue #556: Add reference to IETF cross device security doc (openid/fapi)

[josephheenan]

New issue 556: Add reference to IETF cross device security doc
https://bitbucket.org/openid/fapi/issues/556/add-reference-to-ietf-cross-device

Joseph Heenan:

FAPI-CIBA should probably reference [https://datatracker.ietf.org/doc/draft-kasselman-cross-device-security/](https://datatracker.ietf.org/doc/draft-kasselman-cross-device-security/)