add issue template for missing or incorrect documentation

this allows anyone to create labelled issues for easy filtering.

.github/ISSUE_TEMPLATE/missing_documentation.md

@@ -0,0 +1,28 @@
+---
+name: Missing or incorrect documentation
+about:
+title: ''
+labels: 'documentation'
+assignees: ''
+
+---
+
+## Problem
+
+<!-- describe your problem -->
+
+## Checklist
+
+<!-- make sure this issue is not redundant or obsolete -->
+
+- [ ] checked [latest Nix manual]\ ([source])
+- [ ] checked [open documentation 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/src
+[open documentation issues and pull requests]: https://github.com/NixOS/nix/labels/documentation
+
+## Proposal
+
+<!-- propose a solution -->
+