Bug 1492047 - [geckodriver] Update issue template to request trace logs as attachment. r=whimboo a=doc

DONTBUILD

--HG--
extra : rebase_source : f29b0b1ff5a885fc8778c5d166bf8e76d15cd8b2
extra : histedit_source : 18d021bd46674ade98afd7ce7dbd1b1a316387cf

testing/geckodriver/ISSUE_TEMPLATE.md

@@ -26,4 +26,6 @@ Error and stacktrace produced by client.
 <!--
 See https://searchfox.org/mozilla-central/source/testing/geckodriver/doc/TraceLogs.md
 for how to produce a trace-level log.
+
+For trace logs with more than 20 lines please add its contents as attachment.
 -->