Add use of annotation registry to docs

[andig]

As new user of plain JMS without Symfony I've struggled with various annotation exceptions.

It would be great if the docs could reference the need for setting the annotation registry up (https://stackoverflow.com/questions/14629137/jmsserializer-stand-alone-annotation-does-not-exist-or-cannot-be-auto-loaded):

AnnotationRegistry::registerLoader('class_exists');

[goetas]

Will be happy to accept a pull request about it

[goetas]

done in #1047