github: add an issue template

Modelled after Go's own issue template,
with some tweaks to be more relevant to our obfuscator tool.

For #219.

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,39 @@
+<!--
+Please answer these questions before submitting your issue. Thanks!
+-->
+
+### What version of Garble and Go are you using?
+
+<pre>
+$ garble version
+
+$ go version
+
+</pre>
+
+### What environment are you running Garble on?
+
+<!--
+Please run "go env" in the same directory where you run garble.
+-->
+
+<details><summary><code>go env</code> Output</summary><br><pre>
+$ go env
+
+</pre></details>
+
+### What did you do?
+
+<!--
+Provide clear steps for others to reproduce the error.
+If your code is private, try reproducing via a small program.
+As a fallback, provide as many relevant details as possible.
+-->
+
+
+### What did you expect to see?
+
+
+### What did you see instead?
+
+