feat: Add Subject

subjects/concat2argument/README.md

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+## Concat2Argument
+
+### Instructions
+
+Write a program that concatenates two arguments  and prints the result:
+- If the number of arguments is not 2, print (`'\n'`).
+- don't concatenate the two arguments with space.
+
+### Usage
+
+```console
+$ go run . | cat -e
+$
+$ go run . "Hello" " World!" | cat -e
+HelloWorld!$
+$ go run . " student" talented | cat -e
+talented student$
+$ go run . "Hello" "student" talented ! | cat -e
+$
+```