Update readme to accurately reflect the SimpleForm.browser_validation functionality

I originally forgot to run the simple_form generator: `rails generate simple_form:install`. One of the side effects of not running the generator is you get HTML 5 validation out of the box.

Realising I didn't have a SimpleForm config file, I ran the the generator and my HTML 5 validation notices disappeared due to the default initializer config setting `SimpleForm.browser_validations = false`.

This needs clarification in the the README as it took me a little while to figure out what was going on.