fix typo in README
Add "Not Empty" and "Not Blank" validators

* Being able to test if a value is blank or empty is useful for
  string or array properties
* If a string is empty, that means it does not have any characters.
  * If a string only has whitespace, it is not considered empty
* If a string is blank, that means it does not have any non-whitespace
  characters.
  * This could mean it is either empty (""), or only has whitespace
    characters