README: Explain why the selector constraints in css-blocks exist

People who aren't familiar with BEM don't immediately understand the limitations. We shouldn't require a familiarity with BEM to adopt CSS Blocks.

Start in the README and find the explanation of CSS Blocks selector constraints. Read the surrounding guidance on BEM and update the documentation so the limitations and their purpose are understandable without prior BEM knowledge; the README should make the constraints clear to new adopters.

Written by the indexing model from the issue text.