Add example about assert_schema_conform method in README

[masato-bkn]

Hi, all!

It seems that in committee 5.0.0, the old_assert_behavior became false by default, and assert_schema_conform is no longer experimental. It might be better to describe assert_schema_conform in committee-rails README.

• https://github.com/interagent/committee/blob/master/CHANGELOG.md#500beta1---2023-01-25:~:text=Change%20old%20assert%20behavior%20%23380
• change old assert behavior interagent/committee#380

[willnet]

Thanks!