message: Added checklist and recommendations to issue template.
diff: -<!--
  -If you have a question rather than reporting a bug please go to http://answers.opencv.org where you get much faster responses.
  -If you need further assistance please read [How To Contribute](https://github.com/opencv/opencv/wiki/How_to_contribute).
  -
  -Please:
  -
  -* Read the documentation to test with the latest developer build.
  -* Check if other person has already created the same issue to avoid duplicates. You can comment on it if there already is an issue.
  -* Try to be as detailed as possible in your report.
  -* Report only one problem per created issue.
  -
  -
  -This is a template helping you to create an issue which can be processed as quickly as possible. This is the bug reporting section for the OpenCV library.
  --->
  -
  ##### System information (version)
  <!-- Example
  -- OpenCV => 3.1
  +- OpenCV => 4.2
  - Operating System / Platform => Windows 64 Bit
  -- Compiler => Visual Studio 2015
  +- Compiler => Visual Studio 2017
  -->
  - OpenCV => :grey_question:
  @@ -36,3 +21,42 @@ This is a template helping you to create an issue which can be processed as quic
  ```
  or attach as .txt or .zip file
  -->
  +
  +##### Issue submission checklist
  +
  + - [ ] I report the issue, it's not a question
  + <!--
  + OpenCV team works with answers.opencv.org, Stack Overflow and other communities
  + to discuss problems. Tickets with question without real issue statement will be
  + closed.
  + -->
  + - [ ] I checked the problem with documentation, FAQ, open issues,
  + answers.opencv.org, Stack Overflow, etc and have not found solution
  + <!--
  + Places to check:
  + * OpenCV documentation: https://docs.opencv.org
  + * FAQ page: https://github.com/opencv/opencv/wiki/FAQ
  + * OpenCV forum: https://answers.opencv.org
  + * OpenCV issue tracker: https://github.com/opencv/opencv/issues?q=is%3Aissue
  + * Stack Overflow branch: https://stackoverflow.com/questions/tagged/opencv
  + -->
  + - [ ] I updated to latest OpenCV version and the issue is still there
  + <!--
  + master branch for OpenCV 4.x and 3.4 branch for OpenCV 3.x releases.
  + OpenCV team supports only latest release for each branch.
  + The ticket is closed, if the problem is not reproduced with modern version.
  + -->
  + - [ ] There is reproducer code and related data files: videos, images, onnx, etc
  + <!--
  + The best reproducer -- test case for OpenCV that we can add to the library.
  + Recommendations for media files and binary files:
  + * Try to reproduce the issue with images and videos in opencv_extra repository
  + to reduce attachment size
  + * Use PNG for images, if you report some CV related bug, but not image reader
  + issue
  + * Attach the image as archite to the ticket, if you report some reader issue.
  + Image hosting services compress images and it breaks the repro code.
  + * Provide ONNX file for some public model or ONNX file with with random weights,
  + if you report ONNX parsing or handling issue. Architecture details diagram
  + from netron tool can be very useful too. See https://lutzroeder.github.io/netron/
  + -->
  