Add a message for the reporter to check the master branch before submitting a bug report

Author: 0x2c7

.github/ISSUE_TEMPLATE/bug_report.md

@@ -7,6 +7,14 @@ assignees: ''
 
 ---
 
+Before you submit a bug report, please ensure that the bug is still present with the `master` branch:
+
+```ruby
+gem 'ruby_jard', git: 'https://github.com/nguyenquangminh0711/ruby_jard', ref: 'master'
+```
+
+If it doesn't, please wait for the next release and use the master branch in the mean time. Thank you.
+
 **Describe the bug**
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