Restore the Nix security doc

doc/manual/source/installation/nix-security.md

@@ -0,0 +1,15 @@
+# Security
+
+Nix has two basic security models. First, it can be used in “single-user
+mode”, which is similar to what most other package management tools do:
+there is a single user (typically root) who performs all package
+management operations. All other users can then use the installed
+packages, but they cannot perform package management operations
+themselves.
+
+Alternatively, you can configure Nix in “multi-user mode”. In this
+model, all users can perform package management operations — for
+instance, every user can install software without requiring root
+privileges. Nix ensures that this is secure. For instance, it’s not
+possible for one user to overwrite a package used by another user with a
+Trojan horse.