Add clarification for werft annotations in the PR description

Author: vulkoingim

.github/pull_request_template.md

@@ -33,6 +33,12 @@ Does this PR require updates to the documentation at www.gitpod.io/docs?
 <!--
 Optional annotations to add to the werft job.
 
+If you would like to add additional annotations, each one has to be on a separate line:
+/werft with-preview
+/werft with-payment
+Not:
+/werft with-preview with-payment
+
 * with-preview - whether to create a preview environment for this PR
 -->
 - [ ] /werft with-preview