add an installation guide

svn path=/nixu/trunk/; revision=1240

INSTALL

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+Setting up and running Nix UML ("nixu")
+
+It is possible to run Nix inside a User Mode Linux environment (UML). The Nix
+expressions for this can be found at
+https://svn.cs.uu.nl:12443/repos/trace/nixu/.
+
+
+  Installation on SuSE Linux 9
+
+Download the latest nixpkgs collection and checkout the nixu trunk with
+subversion. Install the uml and uml-utilities packages with Nix and run the
+"make-disk.sh" script which can be found in nixu. After it has built
+everything (or downloaded everything from the nixpkgs cache which can be
+used with nix-pull) you can use the "run.sh" script to start the UML.
+
+
+  Installation on Fedora Core 1
+
+Installing the Nix UML on Fedora Core 1 is a bit tricky. Nix relies on
+Berkeley DB and there are issues with Berkeley DB and NPTL kernels (New POSIX
+Thread Library) on Red Hat based systems. Since the system we build inside
+UML does not use NPTL we cannot use the Nix we use on the host system to
+fill the disk once we've booted our the kernel of our UML.
+
+The solution is as follows:
+
+- install all packages via the nix-pull mechanisms with the MANIFEST from
+  the official nix-pkgs site to avoid that impure builds link against the
+  NPTL glibc on the host system.
+- install Nix in Nix
+- in the nixu scripts edit make-disk.sh and fill-disk.sh.
+  In make-disk.sh prefix all nix-* commands with the absolute path to the
+  host-Nix installation bin directory (for example: /nix/bin). In
+  fill-disk.sh replace all /nix/bin paths with a relative path to the Nix
+  installed with Nix (for example: /usr/home/nix/.nix-profile/bin).
+
+This should fix the installation problems and the Nix UML should boot
+flawlessly.
+
+
+  Nix UML and 2.6 kernel
+
+Installing the current Nix UML packages with a 2.4 kernel on a 2.6
+kernel based host system will most likely fail. Linux kernel 2.6 uses NPTL
+by default, but UML itself seems to be broken somewhat so halfway the system
+seems to hang.
+
+Installing a 2.6 based UML with Nix is not trivial and is not advised.
+
+
+  Adding packages to the UML Nix installation
+
+Packages can be added the the Nix UML installation by editing a few files.
+The first file packages should be added to is 'pkgs.nix'. The other files
+that should be edited all reside in the "boot" directory. In default.nix
+the packages should be added as an argument (first line) and given as an
+argument to the builder (line starting with "inherit"). In "builder.sh"
+there is a giant sed expression which subsitutes all occurences of the
+names of the packages in the files with the right value. If the programs
+that are added need to be added to $PATH the file "env.sh" has to be
+edited.
+
+Another option is to enable networking in the UML and use nix-pull to get
+all the right packages. One thing you have to make sure is that the packages
+you want to install are indeed defined in the nix files.
+
+
+  Differences with a normal UML
+
+A lot of packages are still missing from the current UML. User management
+is completely lacking, init scripts and system configuration (/etc) are
+not there yet.