Update README to make Docker Recommended way (alshedivat#1523)

Docker is well-tested for a long time. I think it is better to make it
the recommended method for all OS types.

This way, it is a lot easier to install docker (than to install jekyll
and ruby and what not) and also we wouldn't have to deal with issues
like these:
- alshedivat#1520
- alshedivat#1491