Update bug_report.md

Author: satoryu

.github/ISSUE_TEMPLATE/bug_report.md

@@ -2,37 +2,39 @@
 name: Bug report
 about: Create a report to help us improve
 title: ''
-labels: ''
+labels: 'bug'
 assignees: ''
 
 ---
 
-**Describe the bug**
+## Describe the bug
+
 A clear and concise description of what the bug is.
 
-**To Reproduce**
-Steps to reproduce the behavior:
-1. Go to '...'
-2. Click on '....'
-3. Scroll down to '....'
-4. See error
+## To Reproduce
 
-**Expected behavior**
-A clear and concise description of what you expected to happen.
+Steps to reproduce the behavior or put a short code to reproduce the bug. 
+
+### example
+
+```rb
+require 'docx'
+
+doc = Docx::Document.new('/path/to/your/docx/file.docx')
 
-**Screenshots**
-If applicable, add screenshots to help explain your problem.
+## Something to reproduce the bug here
+```
 
-**Desktop (please complete the following information):**
- - OS: [e.g. iOS]
- - Browser [e.g. chrome, safari]
- - Version [e.g. 22]
+## Sample docx file
+
+Put a sample docx file to reproduce the bug reported here to this issue. 
+
+## Expected behavior
+
+A clear and concise description of what you expected to happen.
 
-**Smartphone (please complete the following information):**
- - Device: [e.g. iPhone6]
- - OS: [e.g. iOS8.1]
- - Browser [e.g. stock browser, safari]
- - Version [e.g. 22]
+## Environment
 
-**Additional context**
-Add any other context about the problem here.
+- Ruby version: [e.g 2.7.1]
+- `docx` gem version: [e.g 0.5.0]
+- OS: [e.g. iOS]