Deprecate "enum" tag

[nathancastelein]

The new validator package (v9) introduces the oneof tag: https://godoc.org/gopkg.in/go-playground/validator.v9#hdr-One_Of

There is no more need to document the tonic enum tag IMO.

[wI2L]

We kept it so the package remains backward compatible.

[nathancastelein]

Yes, but we should remove its usage from README :)

[wI2L]

@Garbage-Collector I'd rather mark it as deprecated explicitly.