Add find-by-memoization section

[r7kamura]

To avoid a common pitfall, I propose to add a new section about @model ||= Model.find_by(...).

Actually, this was created based on the following cop discussion:

• https://github.com/rubocop/rubocop-rails/pull/1324

[pirj]

I wish Ruby had @foo |||= potentially_nil spikey rose memoisation just for that.