Docs: add examples for encoding

[alexisgauba]

• Add example for simple encoding
• Add example for encoding with custom struct

[ricmoo]

Great idea! Thanks! I'll add this to the docs. :)

[ricmoo]

I'll also add an example using human-readable, e.g.

utils.defaultAbiCoder.encode(
    [
        "(address a, (uint256 b1, bool b2)[] b, uint8 c)"
    ],
    [
        [
            constants.AddressZero,
            [
                { b1: 5, b2: false },
                [ 45, true ]
            ],
            56
        ]
    ]
)

[ricmoo]

Related: #1301 and #1147

[ricmoo]

Added some examples.

Thanks! :)