INSTALL.md: add instructions for using the Nix flake (linera-io#1962)

INSTALL.md

@@ -52,3 +52,35 @@ Alternatively, we have added experimental Nix support (see `flake.nix`).
 * `cargo install cargo-sort`
 * `cargo install cargo-all-features`
 * `cargo install cargo-machete`
+
+# Installation using Nix
+
+Alternatively, this repository contains a Nix flake that can be used
+to prepare a reproducible development environment on Nix-enabled
+systems.
+
+Nix runs on Linux systems (including the Windows Subsystem for Linux)
+and macOS.
+
+If you don't have Nix installed, we recommend using the [Determinate
+Nix installer](https://github.com/DeterminateSystems/nix-installer) to
+easily set up Nix with flakes support.
+
+Alternatively, you can use [the standard
+installer](https://nixos.org/download/) and follow [the
+documentation](https://nixos.wiki/wiki/Flakes) to enable flakes
+manually.
+
+Once Nix is installed, from the repository root, simply run
+
+```shellsession
+$ nix develop
+```
+
+to enter a development environment, and then run `cargo build` or
+`cargo install` normally.
+
+If you have [direnv](https://direnv.net/) installed, there is no need
+to manually run `nix develop`: instead, run `direnv allow` to
+automatically drop into the build environment when you enter the
+project directory.