Adds installation of kramdown to README.

[Jekyll uses the kramdown gem](https://github.com/google/WebFundamentals/blob/master/src/site/_config.yml#L5) for markdown compilation. Without it the project won't build.

Suggestion: it would probably be neater to install all ruby dependencies through a `Gemfile` and a `gem install`, rather than a specific `gem install kramdown`. Would such a pull request be welcome?