Add syntax highlighting to readme file.

[erwald]

For instance, turn this

iex> Nerves.UART.configure(pid, active: true)
:ok

iex> flush
{:nerves_uart, "COM14", "a"}
{:nerves_uart, "COM14", "b"}
{:nerves_uart, "COM14", "c"}
{:nerves_uart, "COM14", "\r"}
{:nerves_uart, "COM14", "\n"}
:ok

into this

iex> Nerves.UART.configure(pid, active: true)
:ok

iex> flush
{:nerves_uart, "COM14", "a"}
{:nerves_uart, "COM14", "b"}
{:nerves_uart, "COM14", "c"}
{:nerves_uart, "COM14", "\r"}
{:nerves_uart, "COM14", "\n"}
:ok

[GregMefford]

@erwald I'm late to the party, but I wanted to say thanks so much for taking the time to make this improvement!

You may think it's a simple change and not a big deal, but it sets a good example for the level of quality we should strive for in all of our documentation. 🎈 🚀 ✨