Docs: Example should append to ignored_columns

[tommyh]

Updating the documentation for setting self.ignored_columns on an ActiveRecord model to append to the existing value instead of overwriting the existing array.

Appending to the existing value, +=, is helpful for the scenario when 2 developers set ignored_columns in different parts of the file and the second one overwrites the first one.

Summary

Our team just hit a production issue where two developers set self.ignored_columns on the same ActiveRecord model, but they did it in different locations in the file. The code looked like this:

class Project < ActiveRecord::Base

  self.ignored_columns = [:category]

  [..... unrelated code here....]

  self.ignored_columns = [:name]

end

So the end result was:

[11] pry(main)> Project.ignored_columns
=> ["name"]

This PR updates the example documentation so that it is more likely developers would write this:

class Project < ActiveRecord::Base

  self.ignored_columns += [:category]

  [..... unrelated code here....]

  self.ignored_columns += [:name]

end

which would result in:

[11] pry(main)> Project.ignored_columns
=> ["category", "name"]

Other Information

There is a rubocop rails style code that recommends the "append" pattern for ignored_columns, but this is a style guide which a rails developer might not know about and/or read.

I will look into adding this to rubocop-rails as well, but that is an optional linter which doesn't have 100% adoption and a documentation fix seems simple enough that everyone can benefit from it.