* Update active_record_basics.md

I made a bit of clarification for people, who are not familiar with SQL (pretty much like me). However, I don't know what tutorial for MySQL is better, so I haven't inserted a link yet.

* [ci skip] For those who new to GitHub

Added more instructions for contributing guides. Without them, it was somewhat confusing for me to find what I should actually do.

* Update active_record_basics.md

Fixed grammar and text wrapping as requested.

* Update contributing_to_ruby_on_rails.md

Revised instructions.

* Update contributing_to_ruby_on_rails.md

Typos

* Update active_record_basics.md

* [ci skip] Update active_record_basics

Added a few links to SQL tutorials found on the net.
Also, changed MySQL to SQL (or one of its extensions) - I think that it's a good compromise.

* [ci skip] I think now it's more clear what to do.

* [ci skip] Fixed strings

[Rafael Mendonça França + loothunter1]