Updates map marker graphics file

Updates map marker graphics file:
- Uses slices instead of artboards, because (a) slices stick to the
pixel grid, (b) number of slices has no cap, and (c) slices can export
all graphics at once.
- Adds instruction text to file.
- Adds filetype to readme.
- Removes unused swatches
- Adds default color swatch group for map markers.
Merge pull request #556 from codeforamerica/graphics-file-layer-names

Updates map marker graphics file