Update README.md

Author: internet5

README.md

@@ -29,3 +29,17 @@ fetch('https://raw.githubusercontent.com/internet5/smart-array-to-tree/master/ex
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  ```
+## API
+### `smartArrayToTree(data, [options])`
+Convert a plain array of nodes (with pointers to parent nodes) to a a nested data structure.
+
+#### Parameters
+- **Array** `data`: An array of data
+- **Object** `options`: An object containing the following fields:
+  - `id` (String): An unique node identifier. Default: 'id'
+  - `pid` (String): A name of a property where a link to a parent node could be found. Default: 'pid'
+  - `children` (String): The child node name that you want. Default: 'children'
+  - `firstPid` (String): The parent id of fisrt level node . Default: null
+
+#### Return
+- **Array**: Result of transformation