Change README.md to README

Signed-off-by: gogojesse <gogojesseco@gmail.com>
Add lession00 for Environment Setup.
1) arm-none-eabi toolchain installation
2) Build QEMU

Signed-off-by: gogojesse <gogojesseco@gmail.com>
First bare metal program.
1) A 3-lines assembly program.
2) How to use gdb step by step

Signed-off-by: gogojesse <gogojesseco@gmail.com>
Add tree of directory and description.

Signed-off-by: gogojesse <gogojesseco@gmail.com>