Protocol Buffers Well-Known Types: missing import statement guidance

[HouzuoGuo]

Some (all?) of the well-known types require an import statement to use (e.g. import "google/protobuf/timestamp.proto").

Without the import statements, code generation won't work.

The documentation neglected to mention this fact. Please improve the documentation so that every well known type is accompanied with the import file required for its usage.