[Openid-specs-fapi] Issue #638: Add some more text to Introduction (openid/fapi)

[Nat]

New issue 638: Add some more text to Introduction
https://bitbucket.org/openid/fapi/issues/638/add-some-more-text-to-introduction

Nat Sakimura:

Currently, it only talks about the FAPI 2 family and not specifically on this document. Perhaps adding a paragraph like the one below would help readers. 

FAPI 2.0 is comprised by several documents. At the time of writing, they are: 

* FAPI 2.0 Attacker model
* FAPI 2.0 Security profile
* FAPI 2.0 Message signing

This document specifies the process for a client to obtain sender-constrained tokens from the authorization server, which is supposed to be used in conjunction with identity or session management specifications such as OpenID Connect, which is the assumption for its formal security analysis. 

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