Root project documentation update

`README.md` is rework. Describe project concept description and
mention links. The old readme file build section move to the separate
file `BUILD.md`. It helps to fast read the corresponding file instead of
viewing a `README.md` table of content.

'CONTRIBUTING.md' file contents information to makes gain experience
and contribute in project easier.

'CHANGELOG.md' provide a detailed, chronological history of change
releases.

Signed-off-by: Alexey Gavrilov <alexey.gavrilov@mail.com>

1 files changed, 73 insertions, 0 deletions

diff --git a/BUILD.md b/BUILD.md
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+## Requirements
+
+1. Compiler toolset chose in the toolchain file
+    * GCC cross compiler toolchain (arch-elf-tool - i686-elf-g++ for compiler)
+
+    To build and install from source refer to the [documentation](https://gcc.gnu.org/install)
+
+    * Clang cross compiler toolchain
+    Clang support cross compile out the box.
+
+    You can download *clang* from your package manager.
+
+    Using *apt*:
+
+    ```bash
+    apt-get update && apt-get install clang
+    ```
+
+    Using *pacman*:
+
+    ```bash
+    pacman -Sy clang
+    ```
+
+    Or build from source consider the [documentation](https://clang.llvm.org/get_started.html)
+
+2. Nasm compiler (for x86 only)
+
+    To download from package manager.
+
+    Using *apt*
+
+    ```bash
+    apt-get update && apt-get install nasm
+    ```
+
+    Using *pacman*
+
+    ```bash
+    pacman -Sy nasm
+    ```
+
+3. Grub bootloader (only it is currently supported)
+
+    It is reccommended to build grub from source configured target as chose
+architecture.
+
+    To build grub from source refer to the [documentation](https://www.gnu.org/software/grub/manual/grub/html_node/Obtaining-and-Building-GRUB.html)
+
+## Kernel configuration
+You can configure kernel using cmake -D option. All cmake options build-in are supported
+
+### Options
+Not yet available options
+
+## Build instructions
+
+### On your host environment
+```
+cmake -DCMAKE_TOOLCHAIN_FILE=<FILE> -D<OPTION>=<VALUE> -B build
+cmake --build build
+# Create iso file using *grub-mkrescue*
+# Assume GRUB configured and built to a chose architecture
+mkdir -p iso/os
+cp build/Objective iso/os
+grub-mkrescue -o build/kernel.iso iso
+```
+
+After you can create virtual machine or using emulator to run kernel.
+For qemu and x86 architecture:
+```bash
+qemu-system-i386 -cdrom build/kernel.iso
+```