polish the issues

.github/ISSUE_TEMPLATE.md

-The issue tracker is **ONLY** used for bug report and feature request. 
+The issue tracker is **ONLY** used for bug report and feature request. Keep in mind, please check whether there is an existing same report before your raise a new one.
 
 Any question or RocketMQ proposal please use our [mailing lists](http://rocketmq.apache.org/about/contact/).
 
@@ -28,4 +28,4 @@ Any question or RocketMQ proposal please use our [mailing lists](http://rocketmq
 
 - [sub-task1-issue-number](example_sub_issue1_link_here): sub-task1 description here, 
 - [sub-task2-issue-number](example_sub_issue2_link_here): sub-task2 description here,
-- ...
\ No newline at end of file
+- ...