update readme for 4.0.0

[shmax]

A lot of people are getting confused by the fact that the readme doesn't reflect the current release version. Until #320 gets merged, this will at least put the documentation in sync with the current state of the code.

@bighappyface @digitalkaoz @nwolff @stamster

[j-gao]

For the inline references, we should also be using $jsonValidator->check($jsonToValidateObject, (object)['$ref' => 'file://mySchema']); instead of $jsonValidator->check($jsonToValidateObject, $jsonSchemaObject);? At least it's the first one that's working for me.