docs: Add an FAQ entry for missing attribute (#1122)

docs/user-guide/faq.md

@@ -24,3 +24,24 @@ extraPlugins = [(pkgs.vimUtils.buildVimPlugin {
 ```
 
 The [nixpkgs manual](https://nixos.org/manual/nixpkgs/stable/#managing-plugins-with-vim-packages) has more information on this.
+
+## How do I solve `<name>` missing
+
+When using NixVim it is possible to encounter an error of the type `attribute 'name' missing`, for example it could look like:
+
+```
+       (stack trace truncated; use '--show-trace' to show the full trace)
+
+       error: attribute 'haskell-scope-highlighting-nvim' missing
+
+       at /nix/store/k897af00nzlz4ylxr5vakzpcvh6m3rnn-source/plugins/languages/haskell-scope-highlighting.nix:12:22:
+
+           11|     originalName = "haskell-scope-highlighting.nvim";
+           12|     defaultPackage = pkgs.vimPlugins.haskell-scope-highlighting-nvim;
+             |                      ^
+           13|
+```
+
+This usually means one of two things:
+- The nixpkgs version is not in line with NixVim (for example nixpkgs nixos-23.11 is used with NixVim master)
+- The nixpkgs unstable version used with NixVim is not recent enough.