Add a section about Rekognition on Google Vision documentation

Take the content from here https://github.com/nuxeo/nuxeo-vision/blob/master/README.md and add it under the Installation & Configuration paragraph https://doc.nuxeo.com/nxdoc/nuxeo-vision/#installation-and-configuration

Also About Google Vision: under Nuxeo Platform Configuration, there is no need to obtain a JSON file. A service account credential key must be obtained, however, and added to the nuxeo.conf file as follows:

org.nuxeo.vision.google.key=[key_goes_here]