add maintenance mode warning to issue template

Author: bernhardmgruber

.github/ISSUE_TEMPLATE.md

@@ -1,3 +1,10 @@
+<!--
+Vc is now in maintenance mode and no longer actively developed.
+However, we continue to review pull requests with bugfixes from the community.
+If your issue is trivial to fix, we might be able to address it.
+Otherwise, please provide a pull request in addition to your issue.
+-->
+
 Vc version / revision | Operating System | Compiler & Version | Compiler Flags | Assembler & Version | CPU
 ----------------------|------------------|--------------------|----------------|---------------------|----
                       |                  |                    |                |                     |