go: add «1518. Water Bottles»

Signed-off-by: Matej Focko <me@mfocko.xyz>

go/water-bottles.go

@@ -0,0 +1,16 @@
+package main
+
+func numWaterBottles(numBottles int, numExchange int) int {
+	drank := 0
+
+	for numBottles >= numExchange {
+		canGet := numBottles / numExchange
+
+		drank += canGet * numExchange
+		numBottles -= canGet * numExchange
+
+		numBottles += canGet
+	}
+
+	return drank + numBottles
+}