Relax breaking changes to check to allow existence of parseable changelog w/ version

[ericphanson]

Currently, the check from #586 allows you to mention a changelog or breaking change in the release notes passed to Registrator.jl. I think it would be good to allow you to skip passing anything to Registrator if you have a parseable changelog in a well-known location which contains release notes for the version being registered.

In JuliaDocs/Changelog.jl#11 I am adding changelog parsing to Changelog.jl, and in JuliaDocs/Changelog.jl#12 I propose adding a check there for users to use in CI. But AutoMerge could use the same check, and if there are release notes, to skip the requirement that they are passed to Registrator explicitly.

[DilumAluthge]

if you have a parseable changelog in a well-known location

How are we defining "well-known location"?

Also, how are we defining "parseable"?

[ericphanson]

Parseable = Changelog.parsefile

Well-known: I proposed NEWS, NEWS.md, CHANGELOG.md in JuliaDocs/Changelog.jl#12

Basically this whole thing would depend on the implementation in Changelog.jl, but users can use it too to check if they will pass accroding to it