[Openid-specs-fapi] Issue #571: Document differences between FAPI1advanced and FAPI2 (openid/fapi)

[dgtonge]

New issue 571: Document differences between FAPI1advanced and FAPI2
https://bitbucket.org/openid/fapi/issues/571/document-differences-between-fapi1advanced

Dave Tonge:

e.g. [https://bitbucket.org/openid/fapi/issues/502/is-nonce-required-with-jarm](https://bitbucket.org/openid/fapi/issues/502/is-nonce-required-with-jarm)