docs: add DEBUG.md (#7859)

Author: Hectorhammett

README.md

@@ -57,6 +57,11 @@ foreach ($entityType->getEntities() as $entity) {
 }
 ```
 
+### Debugging
+
+Please see our [Debugging guide](https://github.com/googleapis/google-cloud-php/blob/main/DEBUG.md)
+for more information about the debugging tools.
+
 ### Version
 
 This component is considered GA (generally available). As such, it will not introduce backwards-incompatible changes in