Add some notes on using NiX to develop pandoc.

doc/nix.md

@@ -0,0 +1,26 @@
+---
+title: Using NiX to develop pandoc
+author: John MacFarlane
+---
+
+The source directory contains `shell.nix` and `flake.nix`,
+so if you have NiX installed, you can use either `nix shell`
+or `nix develop` can be used to obtain a shell with pandoc
+dependencies installed.
+
+To set up `direnv` so that the NiX shell is automatically
+activated whenever the directory is entered, add the following
+`.envrc` in the pandoc source directory:
+
+```
+if ! has nix_direnv_version || ! nix_direnv_version 2.2.1; then
+    source_url "https://raw.githubusercontent.com/nix-community/nix-direnv/2.2.1/direnvrc" "sha256-zelF0vLbEl5uaqrfIzbgNzJWGmLzCmYAkInj/LNxvKs="
+fi
+use flake
+```
+
+This uses [nix-direnv] for caching.  See its web page for
+further help.
+
+[nix-direnv]: https://github.com/nix-community/nix-direnv
+