ISSUE_TEMPLATE.md: add note about expectations regarding bug fixing, …

…inspired from QGIS [ci skip]

.github/ISSUE_TEMPLATE.md

@@ -3,6 +3,14 @@
 Questions should go to the gdal-dev mailing list (https://lists.osgeo.org/mailman/listinfo/gdal-dev)
 or other support forums. GitHub issues are for bug reports and suggestions
 for new features.
+
+The GDAL 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
+adressed in a timely manner.
+If this bug report or feature request is high-priority for you, we suggest
+engaging a GDAL developer or support organisation and financially sponsoring a fix.
+
 -->
 
 ## Expected behavior and actual behavior.