chore(gatsby-image): Clarify IntersectionObserver support in README.md

[bjfresh]

The latest Safari releases for Desktop and iOS now include support for IntersectionObserver:
https://caniuse.com/#feat=intersectionobserver

Description

Documentation edits gatsby-image

Related Issues

[lannonbr]

I'm curious if it is needed to comment for people using older versions of safari given the quick turnaround of people upgrading to new versions of MacOS & iOS. For instance here on Apple's App Store support site, about 88% of people on iOS are running at least iOS 12 now. So if there's less and less people on these older versions of iOS / MacOS, should we need to say this for older versions or just comment that if you are supporting IE that you need polyfills

[bjfresh]

I weighed both options, figuring that a March 2019 release was recent enough to warrant the specificity, especially given that there's just 88% support for IntersectionObserver globally. Of course, that'll fade quickly. According to the stats, 3% are still using incompatible versions of Safari. Realistically it's probably enough to simply encourage people to check browser support, and highlight IE as an outlier.

[wardpeet]

I agree with @lannonbr if we can just drop Safari that would be great :)

[bjfresh]

OK, done. Safari is out.

[wardpeet]

Thank you! 🎉

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