add links to FAQ and known issues

.github/ISSUE_TEMPLATE/bug_report.md

@@ -1,3 +1,8 @@
+Please ensure that you do not find an answer before reporting the issue:
+ - in the FAQ: https://github.com/gildas-lormeau/SingleFile/blob/master/faq.md
+ - in the list of known issues: https://github.com/gildas-lormeau/SingleFile#known-issues
+ - in the help page embedded in the extension
+
 ---
 name: Bug report
 about: Create a report to help us improve