docs: help users write more clear bug reports with a template

.github/ISSUE_TEMPLATE/bug_report.md

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+**Describe the bug**
+
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+
+Steps to reproduce the behavior:
+
+1. Have a rack-attack config as follows:
+
+```ruby
+  # E.g.
+
+  Rack::Attack.cache.store = " ... "
+
+  Rack::Attack.throttle(...
+
+  # And so on
+
+```
+
+2. Start the app server
+3. Visit the page with path '....'
+4. See error
+
+**Expected behavior**
+
+A clear and concise description of what you expected to happen.
+
+**Screenshots**
+
+If applicable, add screenshots to help explain your problem.
+
+**Environment information (please complete the following information):**
+
+ - rack-attack version:
+ - ruby version:
+ - rack version:
+ - rails version (if using rails):
+ - rails environment (if using rails): [e.g. development, production, all]
+ - redis gem version (if reporting redis-related issue):
+ - redis server version (if reporting redis-related issue):
+
+**Additional context**
+
+Add any other context about the problem you think can help here.