strlen program for the exam

subjects/strlenprog.en.md

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+## strlenprog
+
+### Instructions
+
+- Write a program that counts the characters of a `string` and that returns that value.
+
+- If the program receives more than one `string` or no arguments must not return anything (see the `Expected output`).
+
+### Expected output :
+
+```console
+student@ubuntu:~/piscine-go/strlenprog$ go build
+student@ubuntu:~/piscine-go/strlenprog$ ./strlenprog "hello"
+5
+student@ubuntu:~/piscine-go/strlenprog$ ./strlenprog
+student@ubuntu:~/piscine-go/strlenprog$
+student@ubuntu:~/piscine-go/strlenprog$ ./strlenprog "hello" "how are you"
+student@ubuntu:~/piscine-go/strlenprog$
+```