Update spring instuctions to suggest eager_load!
Update spring instuctions to suggest eager_load!

A cleaner solution to usage with spring than copying simple cov
start code to config/spring.rb.

According to suggestion here
https://github.com/colszowka/simplecov/issues/381#issuecomment-347651728
and here https://stormconsultancy.co.uk/blog/development/ruby-on-rails/getting-accurate-code-coverage-metrics-from-simplecov-rails-project/

colszowka/simplecov,colszowka/simplecov