add GitHub issue template

fixes #1

PiperOrigin-RevId: 715769130
Change-Id: I8919cef9e9f9a5be69d5f87e249ebf285ac918dc

Author: stapelberg

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,29 @@
+<!--
+
+Please fill in the following sections.
+
+-->
+
+## open2opaque version
+
+<!-- Replace this line with the output of `open2opaque version`. -->
+
+## Observed Behavior
+
+<!-- Replace this line with a description of what you are seeing. -->
+
+## Expected Behavior
+
+<!-- Replace this line with what you expected instead. -->
+
+## Reproduction Instructions
+
+<!--
+
+Replace this section with detailed step-by-step instructions for how to reproduce your problem:
+Imagine you were sitting at a brand new computer.
+Which commands would you need to enter to make the problem happen?
+You can assume that Go, protobuf and Go Protobuf are all installed.
+(Tip: You can create a new user account on your computer to try it out!)
+
+-->