use gh-pages for publishing vignettes

[vsimko]

related links:

• https://gist.github.com/cobyism/4730490
• http:https://rmflight.github.io/posts/2014/11/travis_ci_gh_pages.html

[vsimko]

Here is how I did it:

• created empty gh-pages branch (branched from initial commit)
• added this branch as a git submodule into vignettes/gh-pages

> git submodule add -b gh-pages https://github.com/vsimko/rroad.git vignettes/gh-pages

• added Gemfile to gh-pages with the following content:

source 'https://rubygems.org'
gem 'github-pages', group: :jekyll_plugins

• added _config.yml and selected the theme
• now, I was able to build the page locally using jekyll serve
• I created script vignettes/publish.sh which copies Rmd file over to gh-pages and uses knitr to build the index.md markdown file, which is served by github pages as a website.