Add reference about micro/macrotask queues

[armanbilge]

Besides the small change in this PR, I've been pondering this statement.

If you're using ExecutionContext and not using this project, you likely have some serious bugs and/or performance issues waiting to be discovered.

Technically, doesn't this problem apply even when not using an EC? E.g. Scala.js code directly using Promises and setTimeout. Like, all SJS code should use this EC with Future.

[djspiewak]

Technically, doesn't this problem apply even when not using an EC? E.g. Scala.js code directly using Promises and setTimeout. Like, all SJS code should use this EC with Future.

This is true, though in that case you're explicitly using those constructs, so presumably you're aware of the tradeoffs.