Add .github/ISSUE_TEMPLATE.md

Author: rouault

.github/ISSUE_TEMPLATE.md

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+<!--
+IMPORTANT: Do NOT use GitHub to post any questions or support requests!
+           They will be closed immediately and ignored.
+
+Questions should go to the OpenJPEG mailing list at
+https://groups.google.com/g/openjpeg or other support forums.
+GitHub issues are for bug reports and suggestions for new features.
+
+The OpenJPEG project is made of contributions from various individuals and
+organizations, each with their own focus. The issue you are facing is
+not necessarily in the priority list of those contributors and consequently
+there is no guarantee that it will be addressed in a timely manner.
+
+-->
+
+## Expected behavior and actual behavior.
+
+...to fill...
+
+## Steps to reproduce the problem.
+
+...to fill...
+
+## Operating system
+
+...to fill...
+
+## openjpeg version
+
+...to fill...