update documentation

lib/README.md

@@ -356,6 +356,29 @@ docs:
       returns: Promise<number>
       static: true
 
+    - name: fold
+      since: unreleased
+      desc: |
+        Folds an array into a promise or a value. The array is traversed sequentially and
+
+        The reducer function can return a promise or directly a value. It always received the last resolved value.
+
+        The folding will stop at the first rejection encountered.
+        ```
+      params:
+        - name: list
+          type: "array<T>"
+        - name: reducer
+          desc: The function to call with the accumulated value, the current element from the array and its index.
+          type:
+            kind: function
+            params: "accumulator: U, value: T, index: number"
+            returns: U | Promise<U>
+        - name: initialValue
+          type: "U"
+      returns: U | Promise<U>
+      static: true
+
     - name: each
       since: 3.0.0
       desc: |