Decimal Support [Or Custom Type Support Documentation]

Noticed that :decimal ecto native type is not supported but managed to get it working via a custom ecto type.

Where is the best place to include this? README or module docs?

```elixir
defmodule DecimalType do

@behaviour Ecto.Type
def type, do: :string

def cast(term) when is_binary(term) or is_number(term) do
{:ok, Decimal.new(term)}
end
def cast(_), do: :error

def dump(%Decimal{} = term) do
{:ok, Decimal.to_string(term)}
end
def dump(_), do: :error

def load(term) when is_binary(term) or is_number(term) do
{:ok, Decimal.new(term)}
end
def load(_), do: :error

end
````