docs: specify to use yarn v1 not v2

[JakubKoralewski]

got some errors with yarn v2, so had to downgrade
to even start working, I think it's a good idea
to put it in the README

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[dwelle]

Actually, it seems to be a yarn bug that's been fixed: yarnpkg/berry#2935

[dwelle]

@JakubKoralewski lemme know if you were indeed on yarn <2.4.1 or this is a different bug.

[JakubKoralewski]

By downgrading I lost the ability to tell which v2 exactly I was on, but now I yarn set version berry again which put me on 2.4.2 and the issue is gone.

[dwelle]

Cool. I've changed the readme to v.2.4.2+ because it's unclear whether 2.4.1 works. The issue mentions 2.4.2, but it wasn't in changelog.

[dwelle]

Thanks!