[Openid-specs-fapi] Issue #730: Add reference to security analysis (openid/fapi)

[josephheenan]

New issue 730: Add reference to security analysis
https://bitbucket.org/openid/fapi/issues/730/add-reference-to-security-analysis

Joseph Heenan:

The FAPI2 security analysis linked from [https://openid.net/security-analysis-fapi-2-0-completed/](https://openid.net/security-analysis-fapi-2-0-completed/) includes a security analysis of FAPI-CIBA, so we should include a link to it in the FAPI-CIBA spec.