github: add GitHub issue template

Fixes #913.

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,22 @@
+### Background
+
+Describe your issue here.
+
+### Your environment
+
+* version of `lnd`
+* which operating system (`uname -a` on *Nix)
+* version of `btcd`, `bitcoind`, or other backend
+* any other relevant environment details
+
+### Steps to reproduce
+
+Tell us how to reproduce this issue. Please provide stacktraces and links to code in question.
+
+### Expected behaviour
+
+Tell us what should happen
+
+### Actual behaviour
+
+Tell us what happens instead