docs: describe descendent-search syntax in advanced By README

Author: Cliabhach

src/main/java/io/appium/java_client/pagefactory/README.md

@@ -32,6 +32,21 @@ There's a special array-style syntax for defining predicates in a Class Chain:
 [`value < 0`]
 ```
 
+#### Searching Descendents
+
+If you replace the backticks (`) around a predicate with dollar signs ($), then
+the Class Chain will match against the children, grandchildren, and other
+descendents of each element.
+
+```
+// Find a cell element with the label 'here'
+XCUIElementTypeCell[`label == 'here'`]
+// Find a cell element which contains SOMETHING ELSE that has the label 'here'
+XCUIElementTypeCell[$label == 'here'$]
+```
+
+#### Handling Quote Marks
+
 Most of the time, you can treat pairs of single quotes or double quotes
 interchangably. If you're searching with a string that contains quote marks,
 though, you'll [want to be careful](https://stackoverflow.com/q/14116217).