add doc about code coverage

[ranska]

for weeks I have strongly hesitate to give a chance to phlex. Because slim is perfect for those who love minimal syntax, I want a real reason to do a switch. And Testing the view of course is well but being sure you have the coverage is very important too, especially for long term maintenance. here is an example with an rspec/turnip scenario. I say that because with active_admin arbre you loose coverage if you do arbre partial (with the default config). So I think it's important to mention this very good adventage. I will continue my experimentation now with more confidence. Thanks for all the energie you put in this project.

[joeldrapper]

Good point. I hadn’t thought about this but it’s a big advantage. I love that I can use Rubocop and Ruby LSP too.