docs: Add readme for sumthemall subject

subjects/sumthemall/README.md

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+## sumthemall
+
+### Instructions
+
+Create a program that receives a number of arguments representing whole numbers and displays the sum of the total. If there are not enough arguments or there is an overflow display 0 followed by a new line.
+
+### Usage
+
+```console
+$ go run . 23 34 53 | cat -e
+110$
+$ go run . 1000 2000 3000 4000 | cat -e
+10000$
+```