[Openid-specs-fapi] Issue #731: FAPI2-MS: Add reference to security analysis (openid/fapi)

[josephheenan]

New issue 731: FAPI2-MS: Add reference to security analysis
https://bitbucket.org/openid/fapi/issues/731/fapi2-ms-add-reference-to-security

Joseph Heenan:

The security analysis [https://openid.net/security-analysis-fapi-2-0-completed/](https://openid.net/security-analysis-fapi-2-0-completed/)  explicitly included checking the non-repudiation parts of FAPI2 message signing I believe, so we should probably add a link to it in the specification.