[Openid-specs-fapi] Issue #616: Add section about errata changes (openid/fapi)

[josephheenan]

New issue 616: Add section about errata changes
https://bitbucket.org/openid/fapi/issues/616/add-section-about-errata-changes

Joseph Heenan:

We should add a section to the FAPI1 specs briefly mentioning the various changes made in the errata update. In particular we need a sentence explaining the Financial-grade → FAPI name change.