.github: Add issue template

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,34 @@
+**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
+
+* Prometheus version: 
+
+	insert output of `prometheus -version` here
+
+* Alertmanager version:
+
+	insert output of `alertmanager -version` here (if relevant to the issue)
+
+* Prometheus configuration file:
+```
+insert configuration here
+```
+
+* Alertmanager configuration file:
+```
+insert configuration here (if relevant to the issue)
+```
+
+* Logs:
+```
+insert Prometheus and Alertmanager logs relevant to the issue here
+```