Move errata from wiki to repository

[ThisIsMissEm]

Currently the errata for the ActivityPub specification are all in the wiki (linked to from the published specification), to simplify merging erratum into the errata, and to align with the ActivityStreams repository, we should move the Errata into a ERRATA.md file in the root of this repository.

Then proposed erratum would be pull requests against that file.

The existing wiki page should be kept but redirect users to the file in this repository.