chore(utils): add async utils doc

docs/docs/core/utils/async.md

@@ -0,0 +1,40 @@
+---
+title: Async utils functions
+tags:
+  - utils
+---
+
+# Async utils
+
+## Summary
+
+Async utils are grouping all function that interact with async stuff like Promises.
+
+## Detailed design
+
+Available functions:
+
+- pipeAsync
+- mapAsync
+- reduceAsync
+
+[See API reference](../../api/Utils) (TODO)
+
+### When to use
+
+Everytime the code has to act with promises and iterate other them, an async utils function should be used.
+
+### Should I add my function here ?
+
+Any util function that manipulates promises can be included in this utils section.
+
+Please consider the next point if a lots of functions are available in the async section.
+
+## Potential improvements
+
+If we begin to use lots of async utils function, we may consider to migrate to a specialized library like [asyncjs](http://caolan.github.io/async/v3/)
+
+## Resources
+
+- [Async file in Strapi](https://github.com/strapi/strapi/blob/9b36c3b10adaa00fd3596853abc63122632c36fe/packages/core/utils/lib/async.js)
+- http://caolan.github.io/async/v3/