Update README for changes in cbor v2.5.0 (#427)

Improve security comparison by adding example decoding 181 bytes with encoding/gob and getting fatal "out of memory" error.

Add example encoding 3-level nested Go struct with fxamacker/cbor to 1 byte CBOR.

Update and clarify "Versions and API Changes" section.

Update "Status" section to mention extended 2.5.0-beta -> 2.5.0 period.

Add to Quick Start:  "NOTE: Unmarshal returns ExtraneousDataError if there are remaining bytes, but new funcs UnmarshalFirst and DiagnoseFirst do not."