TransformEntriesToType: readme extended, fixed small issue

[GordonApplepie]

@TimBeyer as requested I extended the README.md. You will find the problem with the fetcher.ts also in this PR. Due to that change it also made sense to remove the method isEntryTransformToType from the intend interface.

The problem is that we need the entries of all content types since we are tryíng to update the parental entries and replace the old linked entries to the newly created ones. The parental entries can be of every possible type in the space.

The following filter prohibits this even when the id collection is empty:

const filter = {
    'sys.contentType.sys.id[in]': ids.join(','),
    'sys.archivedAt[exists]': 'false'
}

[TimBeyer]

Could you separate the PRs so that we have one clean PR branched from master that does the readme change, and then we can fix the ID filter separately.

[GordonApplepie]

@TimBeyer yeah. true that was not so clever actually. I also saw that it is fixed already 😃 will close down this PR and create a new one for the readme file.