add delete instructions

Author: PatrickFarley

articles/cognitive-services/form-recognizer/quickstarts/label-tool.md

@@ -139,6 +139,7 @@ Next, you'll create tags (labels) and apply them to the text elements that you w
     > * Label values as they appear on the form; don't try to split a value into two parts with two different tags. For example, an address field should be labeled with a single tag even if it spans multiple lines.
     > * Don't include keys in your tagged fields—only the values.
     > * Table data should be detected automatically and will be available in the final output JSON file. However, if the model fails to detect all of your table data, you can manually tag these fields as well. Tag each cell in the table with a different label. If your forms have tables with varying numbers of rows, make sure you tag at least one form with the largest possible table.
+    > * To delete an applied tag, select the rectangle on the document view and press the delete key.
 
 ![Main editor window of sample labeling tool](../media/label-tool/main-editor.png)