Change annotation to field options in readme to align with official protobuf documentation.

[F21]

In the official documentation, the annotations are called field options:

• https://developers.google.com/protocol-buffers/docs/reference/proto3-spec#normal_field
• https://developers.google.com/protocol-buffers/docs/proto3#options

Changing annotation to field options makes the terms being used consistent.

[mwitkow]

Cheers :)