Added migration guide

Author: nicolas-van

version-1-to-2-guide.md

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+
+# Migration guide to upgrade from version 1 to version 2
+
+## Explanation
+
+In version 2.X, modern-async had a major re-design of the API. This re-design occured with the following goals:
+
+* Avoiding function naming conflict with popular libraries (notably lodash)
+* Simplifying the API by removing duplicate functions
+* Proposing safer default parameters regarding concurrency handling
+
+It can be noted that version 1.0 will still be supported for a reasonable amount of time regarding bug fixes.
+
+## How to migrate ?
+
+In order to migrate from version 1.X to version 2.X you should check the following points:
+
+* Most functions were renamed to add an `async` prefix. Example: `map` => `asyncMap`.
+* All functions that had three variants ("normal", `Limit` and `Series`) were simplified to have only 1 function. That function takes a `queueOrConcurrency` argument like the old `Limit` variant, except that argument now has a default value of `1`. Here is how to change these function calls:
+    * If you used the `Limit` variant, just rename it to the new name. The arguments will remain compatible. Example: `mapLimit(a, b, concurreny)` => `asyncMap(a, b, concurreny)`.
+    * If you used the `Series` variant, also rename it to the new name. The arguments will also remain compatible. Example: `mapSeries(a, b)` => `asyncMap(a, b)`.
+    * If you used the "normal" variant (with unlimited concurrency), you must pass `Numbers.POSITIVE_INFINITY` as the `queueOrConcurrency` argument. Example: `map(a, b)` => `asyncMap(a, b, Number.POSITIVE_INFINITY)`. (You should also consider not using infinite concurrency anymore as it tends to be a bad idea in most applications.)