Describe utils.register_sharding_key() call as mandatory or make it unnecessary

[akudiyar]

Currently without calling the function utils.register_sharding_key() users are seeing a strange error "bucket_id is used for sharding, but there's no space...". Although the usage of the function is described at the end of the README, it is not clear for users that calling this function is mandatory for every sharded space.

Possible solutions:

• Automatically detect bucket_id key in the space schema and establish a convention that this key is used for sharding, if no other is specified;
• More clearly describe that function utils.register_sharding_key is mandatory in the README.