Consider appending to ignored_columns as a best practice

[silva96]

Hi, I was adding an ignored column and Rubocop Rails mentioned we better append to them instead of assigning.

Here's the ruby style guide that explains why.

https://rails.rubystyle.guide/#append-ignored-columns

We could suggest appending in the readme instead of assigning,

What do you think?

[ankane]

Hi @silva96, thanks for the suggestion! Updated in the commit above.