[Openid-specs-ab] Issue #2123: Describe Federation principles in Introduction (openid/connect)

[mbj]

New issue 2123: Describe Federation principles in Introduction
https://bitbucket.org/openid/connect/issues/2123/describe-federation-principles-in

Michael Jones:

In #2122, @{557058:471f911d-0ada-4c0f-a97a-687a20089df7} wrote this:

> When an entity enters into a federation it must accept and abide by the rules that the federation has specified. At the same time the federation must behave in a predictable way and of course follow the federation rules and the behaviour specified in the OpenID federation document. It’s a mutual agreement.
>
> One of the cornerstones of the OpenID federation document is that each entity's view matters. For instance an intermediate may want to further restrict the metadata of its subordinates compared to what its superiors defines and it must be allowed to do so.

We should add these principles to the Introduction.  They’re important for all to understand.