Addition of why Rails Migrations is called Migrations

[dstavis]

http://en.wikipedia.org/wiki/Schema_migration

The addition of a link to this article would be a great addition to the guide. Migrations was such a strange name to me that I had trouble focusing on learning how to use it. Learning about the origin of the term, and the in-depth description of how the concept of Schema Migrations is linked to what Rails migrations actually do made my understanding of the concepts complete, concrete, sensible, and obvious.

Please add a sentence on Number 8 of the Intro to Active Record guide, and/or the beginning of the Migrations guide here: http://guides.rubyonrails.org/v3.2.13/migrations.html

Thank you for making these guides so great!!!

[dmathieu]

Would you be willing to open a pull request adding this?
You can find the migrations guide content here: https://github.com/rails/rails/blob/master/guides/source/migrations.md

[senny]

@dstavis link added.