Add README

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>

README.md

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+# Pygos GNU/Linux
+
+## Overview
+
+This directory contains the build system for Pygos GNU/Linux, a small, heavily
+customizable operating system for embedded devices and other special purpose
+systems.
+
+The build system generates a compressed, read only root filesystem image, boot
+loader, kernel binary and a setup script for installing the system on the target
+board, all bundled up in a release tar ball.
+
+An additional disk partition or UBI volume is created for mutable configuration
+files with overlay mount points for directories like /etc or /var/lib.
+
+For details on the Pygos file system layout, see
+[docs/filesystem.md](docs/filesystem.md).
+
+An automated firmware update mechanism is currently still in development.
+For details, see [docs/update.md](docs/update.md).
+
+The build system in this directory downloads the necessary source packages,
+builds a cross compiler toolchain and compiles the entire system from scratch
+for the intended target.
+
+For details on how the build system works or how to add or configure packages,
+see [docs/build.md](docs/build.md).
+
+For an overview of the available documentation see [docs/index.md](docs/index.md).
+
+
+## Target configuration
+
+The Pygos build system is driven by toolchain and package configurations
+that are divided into three categories:
+
+ - Common settings shared by all targets
+   - Contains things like a bare minimum default package set
+ - Board specific settings
+   - Mostly board specific cross toolchain and kernel configuration
+   - Can add additional packages required for that board
+ - Product specific settings
+   - Specifies a list of boards for which the product can be built
+   - Specifies and configures extra packages needed
+   - Can override board settings and provide configuration files
+     specific to *that* product running on *that* board
+
+### Demo configuration
+
+The system currently contains configurations that allows it to run on the
+following boards:
+
+ - Raspberry Pi 3 (ARM, 32 bit)
+ - pc-engines ALIX board (x86, 32 bit)
+
+The following demo products for those boards are planned, but currently
+still in development:
+
+ - "router" builds for all of the above boards. A DHCP server offers
+   addresses on two of the ALIX boards Ethernet ports or on the
+   Raspberry Pi 3 wireless interface. A DHCP client configures the
+   remaining interface, uses it as default route and does NAT routing.
+   DNS queries are resolved via a local root resolver.
+
+
+## How to build the system
+
+The system can be built by running the mk.sh script as follows:
+
+    mk.sh <board> <product>
+
+This will start to download and builds the cross toolchain and system in the
+current working directory. The command can (and should) be run from somewhere
+outside the source tree.
+
+Once the script is done, the final release package is stored in the following
+location (relative to the build directory):
+
+    deploy/release-<board>/release-<board>.tar.gz
+
+## Directory overview
+
+The build system directory contains the following files and sub directories:
+
+ - board/
+     Contains the board specific configuration files. E.g. board/alix/ holds
+     files specific for the ALIX board.
+ - docs/
+     Contains further documentation that goes into more detailed aspects of
+     the build system and the final Pygos installation itself.
+ - pkg/
+     Contains package build scripts. Each package occupies a sub directory
+     with a script named "build" that is used to download, compile and
+     install the package.
+ - product/
+     Contains product specific configuration files. E.g. product/router/ holds
+     files for the "router" product.
+ - util/
+     Contains utility scripts used by mk.sh
+ - LICENSE
+     A copy of the GNU GPLv3
+ - mk.sh
+     The main build script that builds the entire system
+ - README.md
+     This file
+

docs/build.md

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+# The Pygos Buildsystem

docs/filesystem.md

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+# Pygos Filesystem Layout

docs/index.md

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+# Documentation Index

docs/update.md

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+# Firmware updates