chore(docs): Add Stack documentation

markdown/dev/reference/api/stack/attr/en.md

@@ -0,0 +1,23 @@
+---
+title: Stack.attr()
+---
+
+The `Stack.attr()` method adds an attribute to the stack
+and returns the original stack.
+Setting the third parameter to `true` will replace the value of the
+attribute instead of adding it.
+
+## Stack.attr() signature
+
+```js
+Stack stack.attr(
+  string name,
+  mixed value,
+  bool overwrite = false
+```
+
+If the third parameter is set to `true` it will call
+`this.attributes.set()` instead,
+thereby overwriting the value of the attribute.
+
+<Note>This method is chainable as it returns the Stack object</Note>