This commit is mostly a cherry-pick from `ahtn` at
https://github.com/tmk/tmk_keyboard/pull/255.
These are the changes:
* Adds ACTION_LAYER_ONESHOT
* Adds ONESHOT_TAP_TOGGLE
* Mentions sticky keys in the docs on oneshot.
On most Linux distributions the avr_setup.sh script can be sourced ```. avr_setup.sh``` or under Bash ```source avr_setup.sh```. This will try and detect the appropriate package manager and install the required packages.
