/docs/design_guide: added links to firmware build learning guides for SAMD21 & ESP8266. Changes were placed in the "Adding native modules" section, since that seemed to me the best place based on target audience.
Updated documentation for `delay()` which fixes#243.
This was done to allow greatly granularity when deciding what functionality
is built into each board's build. For example, this way pulseio can be
omitted to allow for something else such as touchio.
