Clarify location of 'versions.json' for the multi-version feature

[Dominik Gresch]

In this part of the docs:

it's unclear whether the versions.json should be at the top-level, or in the release/ directory.

Looking at existing projects, it seems "in the release/ directory" is the correct answer.

[Jorge Martínez]

Thanks for opening this issue, @greschd. I agree with you, description is wrong. Let me fix it.