Add erratum for fragment identifiers per issue #610

[evanp]

Issue #610 notes that there is no defined considerations for fragment identifiers with AS2. This erratum notes the absence, and suggests an adjustment to default to JSON-LD treatment.

[snarfed]

Looks good.

The one thing I immediately thought of is resolution failure, eg when fetching the URL (without fragment) returns 404, or doesn't return AS2, or returns an AS2 object that doesn't contain an element with the fragment id. (These are all common on the fediverse right now.) Some examples in w3c/activitypub#367 (comment) and further down the thread.

I expect this doesn't change our resolution failure logic at all, so we probably don't need to say anything explicitly, but I figured I'd check.