Further work on Github usability (issue templates)

Make issue template more obvious about importance of
searching the issue tracker first, and wrap logs appropriately.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #5542

.github/ISSUE_TEMPLATE.md

@@ -1,9 +1,14 @@
 <!--
 Thank you for reporting an issue.
 
-This issue tracker is for bugs and issues only.
-If you need general support please use 
-our mailing list. https://github.com/zfsonlinux/zfs/wiki/Mailing-Lists
+*IMPORTANT* - Please search our issue tracker *before* making a new issue.
+If you cannot find a similar issue, then create a new issue.
+https://github.com/zfsonlinux/zfs/issues 
+
+*IMPORTANT* - This issue tracker is for *bugs* and *issues* only.
+Please search the wiki and the mailing list archives before asking 
+questions on the mailing list.
+https://github.com/zfsonlinux/zfs/wiki/Mailing-Lists
 
 Please fill in as much of the template as possible.
 -->
@@ -30,6 +35,10 @@ modinfo spl | grep -iw version
 
 ### Include any warning/errors/backtraces from the system logs
 <!-- 
+*IMPORTANT* - Please mark logs and text output from terminal commands 
+or else Github will not display them correctly. 
+An example is provided below.
+
 Example:
 ```
 this is an example how log text should be marked (wrap it with ```)