docs: Clarify return value of mixed.test (#1089)
docs: Clarify return value of mixed.test (#1089)

Context:

While it may seem obvious to many, I've previously found myself searching the docs more than once for
whether or not I wanted to return `true` or `false` in `mixed.test` validators. This is a minor change to
the README that clarifies the return value.

Additional Details:

Cleaned up some long lines and capitalized the beginning of each paragraph
jquense/yup,jquense/yup