Was the coolant reservoir completely empty before you refilled and is the level still reducing?
How does the engine run generally since that warning came on?
Ok..if your reservior was empty then its likely the engine light is on maybe due to the Coolant Temperature sensor..as that generally senses the coolant temperature to feed info back to the ecu to alter faul/air mixture according to how hot or cold the engine is..so if the system is empty as such then it may wouldnt be able to do as above...you'd only know that once you get the actual fault codes when you get it plugged in or clear the code(s) and then see if they come back or not...But as far as your coolant problem id say that would be priority on diagnoses to find out why that coolant has dropped..so a coolant pressure test on the system plus as dye sniff test on the coolant system would be carried out..to see if you have external leak somewhere on the coolant system unless you can spot it already when you follow and check all the pipes inc the radiator..Or an internal leak as per headgasket issues..I do hope this helps
The resorvior is generally an overflow so putting coolant into the overflow wouldnt severve any purpose - as long the radiator itself is topped then its fine its just there to serve purpose when the coolant expands when the engine/system is upto temperature - but on the radiator cap spring part itself the overflow rubber washer lifts under pressure to let coolant out to the overflow tank. The inlet to the overflow tank has a syphon which dips into the coolant so it can be sucked back as your system cools down...