[doc] clarify sharding_key parameter

In readme there's an example of space definition with such code:
```
sharding_key = {'customer_id'},
```
However it's not clear at all what this parameter does and how it is related to bucket_id. 
Please provide more information about the parameter.

Is this definition correct for space with a bucket_id field also defined ? 
When is it needed to redefine the sharding key ?