doc: Update 'README'.

* README: Update intro a bit.
(Installing Guix as non-root): Remove as it's largely outdated.
(Contact): Update.

README

@@ -10,8 +10,8 @@ It provides [[https://www.gnu.org/software/guile/][Guile]] Scheme APIs, includin
 domain-specific languages (EDSLs) to describe how packages are to be
 built and composed.
 
-A user-land free software distribution for GNU/Linux comes as part of
-Guix.
+GNU Guix can be used on top of an already-installed GNU/Linux distribution, or
+it can be used standalone (we call that “Guix System”).
 
 Guix is based on the [[https://nixos.org/nix/][Nix]] package manager.
 
@@ -75,37 +75,17 @@ the promise of a build; it is stored as a text file under
 `derivation' primitive, as well as higher-level wrappers such as
 `build-expression->derivation'.
 
-Guix does remote procedure calls (RPCs) to the Guix or Nix daemon (the
-=guix-daemon= or =nix-daemon= command), which in turn performs builds
-and accesses to the Nix store on its behalf.  The RPCs are implemented
-in the (guix store) module.
-
-* Installing Guix as non-root
-
-The Guix daemon allows software builds to be performed under alternate
-user accounts, which are normally created specifically for this
-purpose.  For instance, you may have a pool of accounts in the
-=guixbuild= group, and then you can instruct =guix-daemon= to use them
-like this:
-
-  $ guix-daemon --build-users-group=guixbuild
-
-However, unless it is run as root, =guix-daemon= cannot switch users.
-In that case, it falls back to using a setuid-root helper program call
-=nix-setuid-helper=.  That program is not setuid-root by default when
-you install it; instead you should run a command along these lines
-(assuming Guix is installed under /usr/local):
-
-  # chown root.root /usr/local/libexec/nix-setuid-helper
-  # chmod 4755 /usr/local/libexec/nix-setuid-helper
+Guix does remote procedure calls (RPCs) to the build daemon (the =guix-daemon=
+command), which in turn performs builds and accesses to the store on its
+behalf.  The RPCs are implemented in the (guix store) module.
 
 * Contact
 
 GNU Guix is hosted at https://savannah.gnu.org/projects/guix/.
 
-Please email <bug-guix@gnu.org> for bug reports or questions regarding
-Guix and its distribution; email <gnu-system-discuss@gnu.org> for
-general issues regarding the GNU system.
+Please email <help-guix@gnu.org> for questions and <bug-guix@gnu.org> for bug
+reports; email <gnu-system-discuss@gnu.org> for general issues regarding the
+GNU system.
 
 Join #guix on irc.freenode.net.