message: Added PR readiness checklist.
diff: -<!-- Please use this line to close one or multiple issues when this pullrequest gets merged
  -You can add another line right under the first one:
  -resolves #1234
  -resolves #1235
  --->
  +### Pull Request Readiness Checklist
  -### This pullrequest changes
  +See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request
  -<!-- Please describe what your pullrequest is changing -->
  +- [ ] I agree to contribute to the project under OpenCV (BSD) License.
  +- [ ] To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
  +- [ ] The PR is proposed to proper branch
  +- [ ] There is reference to original bug report and related work
  +- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  + Patch to opencv_extra has the same branch name.
  +- [ ] The feature is well documented and sample code can be built with the project CMake
  