doc: add example for table creation with schema
doc: add example for table creation with schema

Practically, tables created without schema is not very useful.
Adding an example how to infer schema automatically from a type
during table creation.

Change-Id: I1795a0f6285859fe43ab09bdd839328930ab5f87
Reviewed-on: https://code-review.googlesource.com/9450
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go