doc: Mark binary deployment as not implemented yet.

* doc/guix.texi (Features): Add a footnote saying that binary deployment
  is missing.

doc/guix.texi

@@ -429,8 +429,11 @@ scripts, etc.  This direct correspondence allows users to make sure a
 given package installation matches the current state of their
 distribution.
 
+@c FIXME: Remove footnote when it's implemented.
 This foundation allows Guix to support @dfn{transparent binary/source
-deployment}.  When a pre-built binary for a @file{/nix/store} path is
+deployment}@footnote{This feature is not implemented as of version
+@value{VERSION}.  Please contact @email{bug-guix@@gnu.org} for more
+details.}.  When a pre-built binary for a @file{/nix/store} path is
 available from an external source, Guix just downloads it; otherwise, it
 builds the package from source, locally.