Documentation and cleaning up the publishing process

[julienw]

This adds some documentation about the publishing process.
This also changes the script prepublishOnly to prepack when publishing. Both are run currently so it's really a no-op, but I've noticed that more recent versions of yarn only does prepack, so this looks more correct.
I also updated the content of this script to make sure the dependencies are uptodate before building.

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[canova]

Looks good to me. I don't remember having to disable the branch protection part. So it's good to have this. I was thinking if we can do the same thing without it, but probably this is easier to do.
I usually push my tags with git push upstream --tags without the HEAD so it only pushes the tags, but I forgot the commit that publish generates. We still need to push it somehow.