documentation: Add a contributing & maintainers file

Per our devcall last week.  Adding now mainly so we can start pointing
LogAnalyzer fixes to @chapman and quickly get them into master.

@tridge & @randy - I kinda guessed at the maintainers list, we can
update later as needed.
all: Change the editorconfig so that it won't want to reformat

.editorconfig is a great way to ensure developers follow our
line ending/tab conventions, but we shouldn't have it default
to fixing any lines that were not edited.
Copter: Ch7/8 option to retract mount

-Define AUX_SWITCH_RETRACT_MOUNT in defines.h
-Added 27=Retract Mount to Parameters.pde CH7 and CH8
-added int on CH7/8opt AUX_SWITCH_RETRACT_MOUNT
-Added Case #if Mount == ENABLE to set mount mode
made set_mode public in AP_Mount.h lib
Mount: set_mode method made public
Compass: add DEV_ID param and configured method

These allow checking the offsets were created with the current compass
device.
Compass: add dev_id for PX4

dev_id is retrieved from PX4Firmware via ioctl call
Compass: add dev_id for VRBrain
Copter: pre-arm check uses compass configured method
GCS_MAVLink: add second mag to PREFLIGHT_SET_SENSOR_OFFSETS

Updates the comments for parameter 1 only, this does not affect the
message format.
GCS_MAVLink: generate after adding second mag to PREFLIGHT_SET_SENSOR_OFFSETS
GCS_MAVLink: add deprecated note to SET_MAG_OFFSETS
GCS_MAVLink: generate after adding deprecated note to SET_MAG_OFFSETS
Compass: save_offsets accepts compass instance
Compass: set_offsets modified to also save offsets
Compass: add learn_offsets_enabled accessor method
Compass: example sketch update for set_and_save_offsets
Copter: only save compass offsets when learning is on
Copter: PREFLIGHT_SET_SENSOR_OFFSETS used to set compass offsets

remove deprecated ardupilot specific SET_MAG_OFFSETS
Plane: rename compass set_and_save_offsets
Plane: PREFLIGHT_SET_SENSOR_OFFSETS used to set compass offsets

replaces ardupilot specific SET_MAG_OFFSETS message