add slack notification

patches/pantheon-yml.patch

 diff --git a/pantheon.yml b/pantheon.yml
-new file mode 100644
-index 0000000..25f0f6d
---- /dev/null
+index 181e16c..60395d9 100644
+--- a/pantheon.yml
 +++ b/pantheon.yml
-@@ -0,0 +1,5 @@
-+api_version: 1
+@@ -3,3 +3,10 @@ api_version: 1
+ php_version: 5.3
+ 
+ drush_version: 7
 +
-+php_version: 5.3
-+
-+drush_version: 7
++workflows:
++  deploy:
++    after:
++        - type: webphp
++          description: Post to Slack after deploy
++          script: private/scripts/slack_notification.php