Documentation improvements

[liamsi]

Add documentation for the following:

• [ ] time enc / dec differences in binary and JSON
• [ ] add a section about existing attributes / tags (e.g. amino:write_empty)
• [ ] update links: https://github.com/tendermint/go-amino#amino-in-the-wild
• [ ] describe behaviour of *Type vs Type (pointer vs. non-pointer)

[tac0turtle]

• [ ] Document non-extensibility of wire encoded types https://github.com/tendermint/go-amino/issues/34
• [ ] Document and enforce that wire.RegisterInterface requires an exported receiver interface https://github.com/tendermint/go-amino/issues/41

adding these two here and closing the issues to better track all doc changes in one issue