If the old readme needs to be kept around, that's one thing, but I got very confused trying to use MixinExtras before someone pointed out that there's a new readme, whose instructions worked like a charm.
The new readme is linked in large bold text on the release notes of all versions that need it. I thought that would be enough but there's been a few cases of confusion so I'll add a note to the main readme tomorrow.
If the old readme needs to be kept around, that's one thing, but I got very confused trying to use MixinExtras before someone pointed out that there's a new readme, whose instructions worked like a charm.