Polish the issue_template

[RongtongJin]

The issue tracker is used for bug reporting purposes ONLY whereas feature request needs to follow the RIP process. To avoid unnecessary duplication, please check whether there is a previous issue before filing a new one.

It is recommended to start a discussion thread in the mailing lists in cases of discussing your deployment plan, API clarification, and other non-bug-reporting issues.
We welcome any friendly suggestions, bug fixes, collaboration, and other improvements.

Please ensure that your bug report is clear and self-contained. Otherwise, it would take additional rounds of communication, thus more time, to understand the problem itself.

Generally, fixing an issue goes through the following steps:

1. Understand the issue reported;
2. Reproduce the unexpected behavior locally;
3. Perform root cause analysis to identify the underlying problem;
4. Create test cases to cover the identified problem;
5. Work out a solution to rectify the behavior and make the newly created test cases pass;
6. Make a pull request and go through peer review;

As a result, it would be very helpful yet challenging if you could provide an isolated project reproducing your reported issue. Anyway, please ensure your issue report is informative enough for the community to pick up. At a minimum, include the following hints:

Polish the issue_template