Added danger button detail to markdown

playbook/app/pb_kits/playbook/pb_button/docs/_button_default.md

@@ -1 +1 @@
-The primary button is used for the most important button on the page. Secondary buttons can be used for actions that are less important. Button links can be helpful for buttons that do not need a lot of attention drawn to them. Disabled buttons are used when you don't want the user to click the button.
+The primary button is used for the most important button on the page. Secondary buttons can be used for actions that are less important. Button links can be helpful for buttons that do not need a lot of attention drawn to them. Disabled buttons are used when you don't want the user to click the button. Danger buttons are used to indicate destructive actions such as deleting.