Update issue and pull request templates

.github/ISSUE_TEMPLATE/bug_report.md

@@ -1,36 +1,51 @@
 ---
 name: Bug report
-about: Create a report to help us improve
+about: Report unexpected or incorrect behaviour
 title: ''
 labels: bug
 assignees: ''
 
 ---
 
-**Describe the bug**
+## Describe the bug
 
-A clear and concise description of what the bug is.
+<!--
+  A clear and concise description of what the bug is.
 
-If you have a problem with a specific package or NixOS,
-you probably want to file an issue at https://github.com/NixOS/nixpkgs/issues. 
+  If you have a problem with a specific package or NixOS,
+  you probably want to file an issue at https://github.com/NixOS/nixpkgs/issues.
+-->
 
-**Steps To Reproduce**
+## Steps To Reproduce
 
 1. Go to '...'
 2. Click on '....'
 3. Scroll down to '....'
 4. See error
 
-**Expected behavior**
+## Expected behavior
 
-A clear and concise description of what you expected to happen.
+<!-- A clear and concise description of what you expected to happen. -->
 
-**`nix-env --version` output**
+## Metadata
 
-**Additional context**
+<!-- Please insert the output of running `nix-env --version` below this line -->
 
-Add any other context about the problem here.
+## Additional context
 
-**Priorities**
+<!-- Add any other context about the problem here. -->
+
+## Checklist
+
+<!-- make sure this issue is not redundant or obsolete -->
+
+- [ ] checked [latest Nix manual] \([source])
+- [ ] checked [open bug issues and pull requests] for possible duplicates
+
+[latest Nix manual]: https://nixos.org/manual/nix/unstable/
+[source]: https://github.com/NixOS/nix/tree/master/doc/manual/source
+[open bug issues and pull requests]: https://github.com/NixOS/nix/labels/bug
+
+---
 
 Add :+1: to [issues you find important](https://github.com/NixOS/nix/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc).