Add README.md file for update-all

Author: netschwarz

content/dart/concepts/map/terms/update-all/README.md

@@ -0,0 +1,55 @@
+---
+Title: '.updateAll()'
+Description: 'Updates all values in a map by applying a function to each key-value pair.'
+Subjects:
+  - 'Computer Science'
+  - 'Code Foundations'
+Tags:
+  - 'Dart'
+  - 'Methods'
+  - 'Map'
+CatalogContent:
+  - 'learn-dart'
+  - 'paths/computer-science'
+---
+
+The **`.updateAll()`** method in Dart updates all values in a map by applying a specified function to each key-value pair. The function receives both the key and its current value, and returns the new value that will replace the existing one.
+
+## Syntax
+
+```dart
+map.updateAll((key, value) => newValue)
+```
+
+- `map`: The map to be updated.
+- `key`: The key in each key-value pair.
+- `value`: The current value associated with the key.
+- `newValue`: The new value to replace the current value.
+
+## Example
+
+In the following example, the `.updateAll()` method is used to increase all scores by 10:
+
+```dart
+void main() {
+  Map<String, int> scores = {
+    'Alice': 80,
+    'Bob': 90,
+    'Charlie': 85
+  };
+  
+  print('Original scores: $scores');
+  
+  // Increase all scores by 10
+  scores.updateAll((name, score) => score + 10);
+  
+  print('Updated scores: $scores');
+}
+```
+
+The above code produces the following output:
+
+```shell
+Original scores: {Alice: 80, Bob: 90, Charlie: 85}
+Updated scores: {Alice: 90, Bob: 100, Charlie: 95}
+```