add ISSUE_TEMPLATE.md (#1036)

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,47 @@
+<!--
+
+    Please do *NOT* ask usage questions in Github issues.
+
+    If your issue is not a feature request or bug report use:
+    https://groups.google.com/forum/#!forum/prometheus-users. If
+    you are unsure whether you hit a bug, search and ask in the
+    mailing list first.
+
+    You can find more information at: https://prometheus.io/community/
+
+-->
+
+**What did you do?**
+
+**What did you expect to see?**
+
+**What did you see instead? Under which circumstances?**
+
+**Environment**
+
+* System information:
+
+	insert output of `uname -srm` here
+
+* Alertmanager version:
+
+	insert output of `alertmanager -version` here
+
+* Prometheus version:
+
+	insert output of `prometheus -version` here (if relevant to the issue)
+
+* Alertmanager configuration file:
+```
+insert configuration here
+```
+
+* Prometheus configuration file:
+```
+insert configuration here (if relevant to the issue)
+```
+
+* Logs:
+```
+insert Prometheus and Alertmanager logs relevant to the issue here
+```