add ISSUE_TEMPLATE

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,26 @@
+### I'm opening this issue because:
+
+- [ ] I'll report a bug
+- [ ] I'll propose a new feature
+
+### Description
+
+### Failure Information (for bugs)
+
+#### Steps to Reproduce
+
+Please provide detailed steps for reproducing the issue.
+
+1. step 1
+2. step 2
+3. you get it...
+
+#### System configuration
+
+- Operating system:
+- Firefox version:
+- Vim-Vixen version:
+
+#### Console logs
+
+Any relevant log in developer tools: