Clarify documentation

[callumlocke]

If you need to support IE8, do you need both versions? Or just the setimmediate-ie8.js?

I'm confused because the IE8 version actually looks smaller so I don't see why the non-IE8 version needs to exist?

[Octane]

If you need to support IE8, do you need both versions? Or just the setimmediate-ie8.js?

You need to use both versions to support IE8 and other browsers. IE8 version must be included first.
If you want all-in-one solution, you can use YuzuJS/setImmediate. It also use the script.onreadystatechange event for IE8.

I'm confused because the IE8 version actually looks smaller so I don't see why the non-IE8 version needs to exist?

Because the postMessage works faster than the appending of a <script> in the DOM tree.
But the use of the postMessage results in a stack overflow in IE8.