Add issue template

This is a mix of what our old new-ticket-form used to suggest:
http://code.stapelberg.de/git/i3-new-ticket/tree/templates/index.html
…and what CONTRIBUTING.md already entailed.

See https://github.com/blog/2111-issue-and-pull-request-templates

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,18 @@
+Output of `i3 --moreversion 2>&- || i3 --version`:
+
+_REPLACE: i3 version output_
+
+URL to a logfile as per http://i3wm.org/docs/debugging.html:
+
+_REPLACE: URL to logfile_
+
+**What I did:**
+
+_REPLACE: e.g. "I’m pressing Alt+j (focus left)"_
+
+**What I saw:**
+
+_REPLACE: e.g. "i3 changed focus to the window ABOVE the current window"_
+
+**What I expected instead:**
+_REPLACE: e.g. "Focus should be on the window to the left"_