Add placeholder for regex field in create event control

static/directives/create-external-notification.html

@@ -40,6 +40,7 @@
 
                   <!-- Regular expression -->
                   <input type="text" class="form-control" ng-model="currentConfig[field.name]" ng-switch-when="regex"
+                         placeholder="{{ field.placeholder || '' }}"
                          ng-required="!field.optional">
 
                   <!-- Value description -->

static/js/services/external-notification-data.js

@@ -29,6 +29,7 @@ function(Config, Features, VulnerabilityService) {
             'help_text': 'An optional regular expression for matching the git branch or tag ' +
                          'git ref. If left blank, the notification will fire for all builds.',
             'optional': true,
+            'placeholder': '(refs/heads/somebranch)|(refs/tags/sometag)'
           }
         ]
       },