Add validation when parsing enum fields.

Invalid values from the wire are silently ignored.
Unlike full/lite, the invalid values are not stored into the
unknown fields, because there's no way to get them out from
Nano's unknown fields without a matching Extension.

Edited README and slightly moved it towards a standalone
section for Nano, independent of the Micro section.

Change-Id: I2c1eb07f4d6d8f3aea242b8ddd95b9c966f3f177