Add list of color formats to the README

[waldyrious]

Also added some blank lines after headings, for consistency, and break a long line at a sentence boundary.

Note: the list of formats was adapted from the code at

ColorPicker/src/ColorPickerWindow.vala

Lines 82 to 87 in 9c9ac86

	color_format_combobox.append_text ("HEX");
	color_format_combobox.append_text ("hex");
	color_format_combobox.append_text ("rgb");
	color_format_combobox.append_text ("rgba");
	color_format_combobox.append_text ("Gdk RGBA");
	color_format_combobox.append_text ("Qt rgba");

I felt the selection I made was a good balance for the README, but let me know if it should be more exhaustive.