Async example

Quote from https://jaredforsyth.com/2017/06/23/when-will-reasonml-be-ready/:

> The OCaml community has some pretty fundamental fragmentations (there are three incompatible versions of the standard library, two incompatible ways to handle async), and documentation is generally pretty sparse. We’re hoping to establish good “batteries included” standard library (that will work for both js and native), but it will take some time.

To JS programmers, async pattern is very common nowadays, we are get used to it. Hence, been said that "there is no standard for async pattern in OCaml" made me worried how to convert my async codes into Reason counterpart.

It would be nice if there is a tutorial about how to do so in OCaml. (e.g. extend the Todo app to send the new item to a simple backend server)

以链接的文章和Todo应用示例为起点；首先确定async模式，以及应用应如何将新项目发送到后端。完成的标准是提供一篇面向Reason/OCaml和JavaScript读者、解释该流程并包含足够操作步骤以便跟随的教程。

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