add ISSUE_TEMPLATE.md (#1036)
add ISSUE_TEMPLATE.md (#1036)


tinytub/alertmanager,tyrken/alertmanager,joehandzik/alertmanager,GoogleCloudPlatform/alertmanager,Kellel/alertmanager,tyrken/alertmanager,joehandzik/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,prometheus/alertmanager,tyrken/alertmanager,tinytub/alertmanager,tyrken/alertmanager,Aloomaio/alertmanager,Aloomaio/alertmanager,Kellel/alertmanager,prometheus/alertmanager,Kellel/alertmanager,joehandzik/alertmanager,Aloomaio/alertmanager,tinytub/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager