Add note for bug reports to check for existing ones

.github/ISSUE_TEMPLATE.md

 Make sure you are running the latest version of Anbox before reporting an issue.
 
+Please also check that no similar bug is already reported. Have a look on the
+list of open bugs at https://github.com/anbox/anbox/issues
+
 **Anbox release (`anbox version`) and system info (`anbox system-info`):**