Add a serialization/deserialization example to the documentation

[pbackus]

From the discussion on #57:

Can you please show example how to serialize sumtype object and deserialize it to original state (for example to and from abstract text representation)? By now I don't see easy (short) way, it requires many non trivial or not universal code (synchronize methods of getting tag, tag type, tag limits, type by tag in serialization and deserialization parts, when it simply hidden by private).