Try this: put print_r($array-or-object, true) as parameter to info.
Alternatively You can log extra data using the second param, ie (for category data)
$object_manager->get('Psr\Log\LoggerInterface')->info("category_data", array('category_data', $category->getData);
But the array will be converted to json which isn't the most "readable" format, so for a basic log I still using
print_r for readability.
BTW: Json output it's perfect if you use "logstash" or another parser.
LOG 4 DUMMIES:
If you are trying to log an entire Quote Object which has a lot of
objects inside (and many recursion too) you must either log an
specific part or use xdebug/zend_debug to handle such "heavy"
data, otherwise you'll probably exhaust the allocated memory for the request.