Add an ISSUE_TEMPLATE.md file (#997)

I'm not good with these, but we have to start somewhere. Feel free to extend
with own ideas. The main point for me here is to ask for the output of `awesome
--version`.

Signed-off-by: Uli Schlachter <psychon@znc.in>

ISSUE_TEMPLATE.md

@@ -0,0 +1,15 @@
+Output of `awesome --version`:
+
+_REPLACE with full version output_
+
+**How to reproduce the issue:**
+
+_REPLACE with step-by-step instructions on what to do_
+
+**Actual result:**
+
+_REPLACE with what happened_
+
+**Expected result:**
+
+_REPLACE with what should happen_