Add text about conformance testing to FAPI2

[Joseph Heenan]

FAPI1 contains some text talking about conformance testing:

https://openid.net/specs/openid-financial-api-part-2-1_0-final.html#incomplete-or-incorrect-implementations-of-the-specifications

We probably want to add something along similar lines to the various FAPI2 specs too.

[Dave Tonge]

We agreed this would be good to have in.

As well as 8.7 we also have some intro text around this

Although it is possible to code an OpenID Provider and Relying Party from first principles using this specification, the main audience for this specification is parties who already have a certified implementation of OpenID Connect and want to achieve a higher level of security. Implementers are encouraged to understand the security considerations contained in Section 8.7 before embarking on a 'from scratch' implementation.

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[Daniel Fett]

PR: https://bitbucket.org/openid/fapi/pull-requests/436

[Dave Tonge]

PR merged