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 +1,32 @@
+---
+name: Missing or incorrect documentation
+about:
+title: ''
+labels: '9.needs: documentation'
+assignees: ''
+
+---
+
+## Problem
+
+<!-- describe your problem -->
+
+## Checklist
+
+<!-- make sure this issue is not redundant or obsolete -->
+
+- [ ] checked [latest Nixpkgs manual] \([source][nixpkgs-source]) and [latest NixOS manual]] \[source][nixos-source])
+- [ ] checked [open documentation issues] for possible duplicates
+- [ ] checked [open documentation pull requests] for possible solutions
+
+[latest Nixpkgs manual]: https://nixos.org/manual/nixpkgs/unstable/
+[latest NixOS manual]: https://nixos.org/manual/nixos/unstable/
+[nixpkgs-source]: https://github.com/NixOS/nixpkgs/tree/master/doc
+[nixos-source]: https://github.com/NixOS/nixpkgs/tree/master/nixos/doc/manual
+[open documentation issues]: https://github.com/NixOS/nixpkgs/issues?q=is%3Aissue+is%3Aopen+label%3A%229.needs%3A+documentation%22
+[open documentation pull requests]: https://github.com/NixOS/nixpkgs/pulls?q=is%3Aopen+is%3Apr+label%3A%228.has%3A+documentation%22%2C%226.topic%3A+documentation%22
+
+## Proposal
+
+<!-- propose a solution -->
+