faq: add instructions on targeting multiple logins

Content adapted from

    https://github.com/rycee/home-manager/issues/394#issuecomment-422958338

FAQ.md

@@ -64,3 +64,43 @@ adding
 
 to your `.profile` and `.zshrc` files, respectively. The
 `hm-session-vars.sh` file should work in most Bourne-like shells.
+
+How do set up a configuration for multiple users/machines?
+----------------------------------------------------------
+
+A typical way to prepare a repository of configurations for multiple
+logins and machines is to prepare one "top-level" file for each unique
+combination.
+
+For example, if you have two machines, called "kronos" and "rhea" on
+which you want to configure your user "jane" then you could create the
+files
+
+- `kronos-jane.nix`,
+- `rhea-jane.nix`, and
+- `common.nix`
+
+in your repository. On the kronos and rhea machines you can then make
+`~jane/.config/nixpkgs/home.nix` be a symbolic link to the
+corresponding file in your configuration repository.
+
+The `kronos-jane.nix` and `rhea-jane.nix` files follow the format
+
+```nix
+{ ... }:
+
+{
+  imports = [ ./common.nix ];
+
+  # Various options that are specific for this machine/user.
+}
+```
+
+while the `common.nix` file contains configuration shared across the
+two logins. Of course, instead of just a single `common.nix` file you
+can have multiple ones, even one per program or service.
+
+You can get some inspiration from the [Post your home-manager home.nix
+file!][1] Reddit thread.
+
+[1]: https://www.reddit.com/r/NixOS/comments/9bb9h9/post_your_homemanager_homenix_file/