Add an introduction to the ruby styleguide

Some words to set out the intent and applicability of the styleguide.
Shamelessly stolen from Python's PEP 8 [1]

[1]
https://www.python.org/dev/peps/pep-0008#a-foolish-consistency-is-the-hobgoblin-of-little-minds
Merge pull request #34 from alphagov/add_ruby_intro_para

Add an introduction to the ruby styleguide