added bookdown.io documentation

[sandrokeil]

I've added the bookdown.io documentation and there are some issues:

1. Some links (license, composer.json, ...) are broken because they are not absolut
2. It is possible to not include the bookdown templates in require-dev, but the default template looks not pretty much. I don't know if users generates the docs by yourself if we provide online docs for gh-pages and link to them in the readme.md file.
3. I did not change the doc structure for BC but I think this is the best way

What do you think?

[sandrokeil]

@codeliner Regarding to our conversation, we can replace the relative links for the gh-pages with an absolute uri by using sed or a bash script and point it to master. This should not be a problem.

1. resolved
2. resolved
3. resolved

Ready to merge. ;-)