Add Issue template

.github/ISSUE_TEMPLATE.md

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+# Please check everything that applies to your issue:
+- [ ] I looked in closed issues and it has not already been answered
+- [ ] My issue appeared with a terminal-notifier update
+- [ ] I'm using a tool that uses terminal-notifier (guard-notifier, node-notifier...)
+
+# To help us debug your issue please include:
+- the macOS version you use
+- terminal-notifier version
+- how did you install terminal-notifier (binary download, homebrew, ruby gem...)?
+- step-by-step reproduction instructions
+
+# Common issues and solution:
+- I'm using tmux -> see #115
+- I'm using iterm2 -> see #147
+- I want to change the icon -> see https://github.com/julienXX/terminal-notifier/issues/197#issuecomment-301305576
+
+Thanks!