* Add an extra prerequisite - ruby2.x-dev
Trying to install Jekyll on my Linux (Ubuntu 16.04) machine failed because ```ruby 2.x``` was installed and ```ruby2.x-dev``` was not. I spent a lot of time trying to figure it out (I'm no ruby expert) but installing the ```ruby2.x-dev``` solved my problem.
* Add a note for Ubuntu users
* Added how-to README.md file to docs/ directory; and referenced
the docs directory in the main README.md.
* Made requested changes from jgthms.
* made requested changes
