Create CONTRIBUTING.md

Added a contributing description, more details to be added. (1st draft)
Merge remote-tracking branch 'upstream/master' into 274
Add support for ArcGIS World Topo maps
Fix the main problem with dataflash downloads: function was expecting time in milliseconds, getting seconds.
Update CONTRIBUTING.md
GITHUB: add link and commit rules to CONTRIBUTING.md
Info Widget: Fixes #389 by making columns expand in response to new data
Merge remote-tracking branch 'upstream/master' into 274
Log Download: Fixes #391. Checks for existing file and if exists appends duplicate number to new file.
MapWidget: Fix #394, MapRipper not closing out after the last zoom level is downloaded
Merge remote-tracking branch 'upstream/master' into 274