Added Two Clarification "Tips"

This PR adds two new documentation "tips" (in guides/source/getting_started.md) to help clarify potentially confusing portions of the guide for brand new Ruby on Rails users:

- An example that illustrates exactly what the `params` method is.
- An explanation of why the `A` in `Article.new` is capitalized for the first time in the guide, whereas it has been lowercase every previous time.

This PR only modifies the Rails "getting started" guide. No code changes have been made.