Add README file

Signed-off-by: David Oberhollenzer <goliath@infraroot.at>

README.md

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+# About
+
+This package contains the source code of a small archiving tool with support
+for dependency management, aka a package manager, developed for the Pygos
+build system.
+
+Building this package produces a program called `pkg` that can:
+
+* take a package description file and a set of input files and pack them to
+  a compressed archive.
+* dump the contents and meta data of such an archive for inspection.
+* install an archive and all its dependencies recursively, in topological
+  order, to a specified root directory.
+* generate file listings from archives in formats suitable for `mksquashfs`
+  and Linux `CONFIG_INITRAMFS_SOURCE`.
+* work out a build order for source packages given information on what source
+  packages provide what binary packages and what binary packages they need in
+  order to build.
+* produce pretty dependency graphs.
+
+## License
+
+The source code in this package is provided under the OpenBSD flavored ISC
+license. So you can practically do as you wish, as long as you retain the
+original copyright notice. The software is provided "as is" (as usual) with
+no warranty whatsoever (e.g. it might actually do what it was designed for,
+but it could just as well set your carpet on fire).
+
+The sub directory `m4` contains third party macro files used by the build
+system which may be subject to their own, respective licenses.
+
+## Package File Format
+
+A writeup on the package file format can be found in
+[doc/fileformat.md](doc/fileformat.md).
+
+The format is currrently not finalized and will be frozen with the 1.0 release
+of this package.
+
+The `pkg` utility has an extensive built in help that can be accessed through
+`pkg help` or `pkg help <command>`.
+
+
+# Usage Example
+
+An example package could be created from the following description file,
+named `foobar.desc`:
+
+	requires coreutils libfrob basefiles
+	toc-compressor zlib
+	data-compressor lzma
+
+In order to be installed, the package requires the other packages `coreutils`,
+`libfrob` and `basefiles` to already be installed beforehand.
+
+For whatever reason, we specify that the table of contents should be compressed
+with zlib and the data with lzma, instead of using the built in default
+choices.
+
+In addition to the description file, we most likely also need want to include
+some files in the package, so we create a file listing `foobar.files`:
+
+	# this is a comment, outlining commentary syntax and explaining
+	# that we create a bunch of directories with permission bits 755,
+	# owned by the user root
+	dir bin 0755 0 0
+	dir dev 0755 0 0
+	dir home 0755 0 0
+
+	# this directory is owned by a different user and group
+	dir home/goliath 0750 1000 1000
+
+	# create a symlink named /root that points to /home/goliath
+	slink root 0777 0 0 /home/goliath
+
+	# create a file /home/goliath/README.md from the input file
+	# README.md stored relative to the foobar.files
+	file home/goliath/README.md 0644 1000 1000 README.md
+
+	# create a file named /bin/foobar. Since we omit the input file path,
+	# assume the file ./bin/foobar also exists relative to foobar.files
+	file bin/foobar 0755 0 0
+
+	# create a character device with permissions 600, owned by 0:0,
+	# device number major 5, minor 1
+	nod dev/console 0600 0 0 c 5 1
+
+Running the following command:
+
+	pkg pack -d foobar.desc -l foobar.files -r ./repodir
+
+Creates a file `./repodir/foobar.pkg` from the package description and file
+listing. Note that the files in the listing are the only things that actually
+have to exist anywhere in the real file system to create the archive.
+
+Lets say, we want to install `foobar` and all its dependencies recursively
+into a staging root directory. Running the following command is sufficient:
+
+	pkg install -r ./rootfs -R ./repodir foobar
+
+This installs `foobar.pkg` inside `./repodir`. Since `foobar` depends on
+`coreutils`, `libfrob` and `basefiles`, the archive files `coreutils.pkg`,
+`libfrob.pkg` and `basefiles.pkg` are also read from `./repodir` and
+installed to `./rootfs`. The same way, all transitive dependencies are
+installed recursively.
+
+Assuming we are not running as root, the above command will tell us that it
+cannot create device nodes while installing and it has trouble changing
+permission/ownership on some file.
+
+So instead, we could run the following command, to keep ownership of the user
+that ran the command, use default permissions and not create device files:
+
+	pkg install -omD -r ./rootfs -R ./repodir foobar
+
+For more information on possible options, simply run `pkg help install`.
+
+Now lets assume we want to pack the staging root directory into a squashfs
+file system.
+
+The `mksquashfs` command is a bit silly in regards to managing user IDs of
+files, but has half-assed support for what it calls "pseudo files" that let
+us set arbitrary user IDs, permissions and create device files, even as a
+regular user.
+
+So we run the following command to generate a squashfs pseudo file for the
+installed packages:
+
+	pkg install -l sqfs -r ./rootfs -R ./repodir foobar > pseudo.txt