[Snek] [keith-packard/snek] 752291: doc: Add section about Arduino Mega board

[Keith Packard]

  Branch: refs/heads/master
  Home:   https://github.com/keith-packard/snek
  Commit: 752291780a0dbc77467625318c17cfb57eb4ac2d
      https://github.com/keith-packard/snek/commit/752291780a0dbc77467625318c17cfb57eb4ac2d
  Author: Keith Packard <keithp at keithp.com>
  Date:   2019-05-01 (Wed, 01 May 2019)

  Changed paths:
    M doc/snek.adoc

  Log Message:
  -----------
  doc: Add section about Arduino Mega board

Describe the pin mappings, PWM functionality and how to flash a device
with firmware.

Signed-off-by: Keith Packard <keithp at keithp.com>