Add keywords.txt

[fpistm]

See: https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5-3rd-party-Hardware-specification#keywordstxt

As of Arduino IDE 1.6.6, per-platform keywords can be defined by adding a keywords.txt file to the platform's architecture folder. These keywords are only highlighted when one of the boards of that platform are selected. This file follows the same format as the keywords.txt used in libraries. Each keyword must be separated from the keyword identifier by a tab.