Add the release dates to RELEASES.md

[kud1ing]

Pages like Wikipedia/Wikidata need to prove that a given information is correct.
The announcement blog posts seem to be the only obvious place to look up the the exact release dates. Blog post URLs are not necessarily stable over time. It would be simpler to just refer to RELEASES.md.

[steveklabnik]

They already contain the month and the year, is the day that important? (that said, I don't see the harm, I was just like "I thought we did this already")

[kud1ing]

is the day that important?

That probably depends on the context. Most software on Wikidata has the complete date defined along the versions. Does it hurt? If the days where given, what could be reasons to remove them?

I would add the days in a PR myself, just wanted to make sure there is no opposition. I suggest we use ISO-format.

The dates are currently defined in Wikdata as https://www.wikidata.org/wiki/Q575650