Added a GitHub issue template.

.github/issue_template.md

@@ -0,0 +1,20 @@
+Please read this text and delete everything up to and including the line of slashes before
+filing your issue. Please
+
+- fill out the table below the line of slashes and keep it at the end of your
+  issue text. The provided answers below are examples, only keep one of them or
+  replace them entirely with something else. But please provide an answer for
+  every line.
+- provide a minimal viable repro case, ideally following
+  https://github.com/SirVer/ultisnips/blob/master/CONTRIBUTING.md#reproducing-bugs.
+- If this is not possible, post a minimal, complete `.vimrc`, snippet
+  definition, and set of key strokes that reproduces your problem.
+- remember to delete this boilerplate text up to and including the line of
+  slashes.
+
+///////////////////////////////////////
+
+- **OS**:  e.g. Windows XP / Ubuntu Linux / Mac OS 10.5
+- **Vim**: e.g. Vim 8.0, Vim 7.4, Neovim 0.3
+- **UltiSnips**: e.g. 3.1, commit 16c999e8c71134401a78d4d46435517b2271d6ac. Do
+  not write `latest master`, please look up the actual revision.