Updates README.md with more inclusive language

[hxlnt]

This change replaces "a sanity check" with "an additional check."

Justification:

The phrase sanity check is ableist, and unnecessarily references mental health in our code bases. It denotes that people with mental illnesses are inferior, wrong, or incorrect, and the phrase sanity continues to be used by employers and other individuals to discriminate against these people.

Via Ableist language

See also: Google developer documentation style guide: Writing inclusive documentation