Add git annex init to download instructions

[mguaypaq]

This matches the download instructions for data-single-subject.

Addresses #113

[kousu]

Right, what an 🍳. Thanks for debugging that :). The CI has this line in it:

data-multi-subject/.github/workflows/validator.yml

Line 97 in 1eb87c0

git annex init

so of course the instructions should too.

I'm still constantly surprised by counterintuitive parts of git-annex. On other datasets I've seen git annex add (or any other git annex command) run git annex init implicitly. But perhaps it somehow does it in a way that manages to avoid special remotes?

The wiki (permalink) doesn't list the init line and hasn't for a while and hasn't caused a problem. I just really don't understand the instinct I'm supposed to have for git annex.

This reminds me of how git annex sync also has a lot of subtle corner cases ( neuropoly/data-management#64 ).