Update issue template (#809)

Author: reese

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,47 @@
+---
+name: Bug Report
+about: Report a formatting issue with rubyfmt
+---
+
+<!--
+
+Hi there! Thanks for taking the time to file an issue against rubyfmt!
+To make filling out this form faster:
+
+1. Create a program showing the bug on https://rubyfmt.run
+3. Click "Gihub" in the header, then "Create issue with example" in the dropdown menu
+
+This will pre-populate most of the form below.
+
+We accept issues primarily of the form "rubyfmt broke this file", or
+"rubyfmt failed to format this file". We define "breaking" as any behaviour
+that the file has changes.
+
+Please don't open "feature request" issues, or "I'd like to change the output to
+be more like this" issues. We've got a clear direction for those and we'll open
+issues up to those in a future release.
+ -->
+
+- `rubyfmt` version/commit: TODO <!--- If unsure, run `rubyfmt --version` --->
+
+[→ View on rubyfmt.run](TODO_replace_this_with_your_rubyfmt_run_link)
+
+## Input
+
+```ruby
+# TODO: In addition to the rubyfmt.run link, replace this comment with your code
+```
+
+## Output
+
+```ruby
+# TODO: Replace this with the output from rubyfmt.run
+```
+
+## Expected behavior
+
+<!-- TODO: Briefly explain what the expected behavior should be on this example. -->
+
+---
+
+<!-- TODO: If there is any additional information you'd like to include, include it here. -->