github: add remark about general questions (ethereum#15250)

Author: bas-vk

.github/ISSUE_TEMPLATE.md

@@ -1,3 +1,9 @@
+Hi there,
+
+please note that this is an issue tracker reserved for bug reports and feature requests.
+
+For general questions please use the gitter channel or the Ethereum stack exchange at https://ethereum.stackexchange.com.
+
 #### System information
 
 Geth version: `geth version`