Add readme file for sumascii subject

subjects/sumascii/README.md

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+## sumascii
+
+### Instructions
+
+Create a program that receives an argument and displays the sum of the ASCII value of each character. The sum will be a number representing the value after the sum of the bytes. If there is an invalid input print 0.
+
+### Usage
+
+```console
+$ go run . "hi" | cat -e
+209$
+$ go run . "" | cat -e
+0$
+```