Improved hex docs

[orestis]

The current hex docs are quite bare: Only a minimum API reference, empty modules (Poison.Encoder, Poison.Pretty) and no examples of what options can be passed to encode etc.

It would be nice to expose the current README in the hex docs, to start with. Thanks!

[zorbash]

I'm willing to submit a PR for this. Currently I'm looking for a way to pretty encode a value using
Poison.encode/2 and the docs

def encode(value, options \\ [])                  

Encode a value to JSON.

┃ iex> Poison.encode([1, 2, 3])
┃ {:ok, "[1,2,3]"}

One shouldn't have to have to look at the code to figure out the available options.

[cthree]

I'm also willing to take the time to write some doc blocks. For such a popular module it has uncharacteristically sparse docs. It's a significant barrier to entry.

[scmx]

I'm also willing to help improving the docs. I see that there's already a related PR open #123