Added note for prioritize/escalate issues via support contract.

.github/ISSUE_TEMPLATE.md

@@ -6,6 +6,8 @@ Hi there - thanks for filling an issue. Please ensure the following things befor
 - Add the `log/production.log` file from your system. Attention: Make sure no confidential data is in it!
 - Please write the issue in english
 
+Note: We always do our best. Unfortunately, sometimes the requests are too much and we can't handle everything at once. If you want to prioritize/escalate your issue, you can do so by means of a support contract (see https://zammad.com/pricing#selfhosted).
+
 * The upper textblock will be removed automatically when you submit your issue *
 -->