Add a link to PS Vita getentropy to the README

[lukateras]

This is a follow-up to #576. Just found the link!

[newpavlov]

Ideally, we would use a link to official docs, but in its absence I guess such link is fine. Though I think it's better to use this link instead.

The CI failure is not relevant, so we can ignore it. It will be fixed in #578.

[lukateras]

Aaa thank you for such a quick turnaround! ^^ (I do somewhat prefer the previous link since it shows the exact revision where it was added, similar to the 3DS link, but the reference is clear one way or the other. )