isBoolean(str [, options]) | check if the string is a boolean.
options is an object which defaults to { loose: false }. If loose is set to false, the validator will strictly match ['true', 'false', '0', '1']. If loose is set to true, the validator will also match 'yes', 'no', and will match a valid boolean string of any case. (e.g.: ['true', 'True', 'TRUE']).

had a typo, fixed in README.md file.