Add mdbook for better reading exprerience

[cgyinthehouse]

Jumping between the content of the table and the snippets are quiet annoying for me when reading the README.md file. Because whenever I click the "Back to Top" anchor and navigate to the table of contents, I have to scroll to the title to find the last topic I read and sometimes I just forget what's the title or the number I just read.

Some github repos uses mdbook to write their tutorials, so I wrote a python script (write_to_files.py in the src directory) to generate a SUMMARY.md and the src files for mdbook. Now I can use the mdbook build or mdbook server command and to read this book more easily.

Mdbook has some useful shortcut and search function and It's much more comfortable compares to reading directly in the README.md.