mirror: Add issue template

Add an issue template to encourage users to report issues to the BPF mailing
list rather than to GitHub Issues. The GitHub issues should be used either to:

- Track issues reported to the mailing list and known to the maintainers
- Report and track issues related to the mirror itself (mirror-specific files
  or modifications)

Signed-off-by: Quentin Monnet <qmo@kernel.org>

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,36 @@
+---
+name: Bug report
+about: Report an issue when building or using bpftool
+---
+
+## Reporting an issue
+
+<!--
+Issues on this repository are used for two purposes:
+
+  1. Track issues that have been reported on the BPF mailing list, and are known to the maintainers
+  2. Report and track issues that are proper to the GitHub mirror (such as mirror-specific files or changes)
+
+Please report new bpftool bugs to the BPF mailing list first. More people will see your report there.
+
+- Mailing list address: bpf@vger.kernel.org
+- Mailing list subscription: http://vger.kernel.org/vger-lists.html#bpf
+- Mailing list archives: https://lore.kernel.org/bpf/
+- BPF reporting documentation: https://docs.kernel.org/bpf/bpf_devel_QA.html#q-how-do-i-report-bugs-for-bpf-kernel-code
+-->
+
+- [ ] I considered whether to report to the BPF mailing list first
+
+## Environment
+
+- bpftool version: 
+- Linux kernel version: 
+- (optionally) clang/LLVM version: 
+- ...
+
+## Describe the bug
+
+<!--
+A clear and concise description of what the bug is.
+Please consider providing reproducing steps, or - when an eBPF program is involved - a minimal reproducing example.
+-->