greatest common divisor exam exercise readme

subjects/gcd.md

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+# gcd
+## Instructions
+
+Write a program that takes two strings representing two strictly positive
+integers that fit in an int.
+
+Display their highest common denominator followed by a newline (It's always a
+strictly positive integer).
+
+If the number of parameters is not 2, display a newline.
+
+## Expected output
+
+```console
+student@ubuntu:~/student/gcd$ go build
+student@ubuntu:~/student/gcd$ ./gcd 42 10 | cat -e
+2$
+student@ubuntu:~/student/gcd$ ./gcd 42 12 | cat -e
+6$
+student@ubuntu:~/student/gcd$ ./gcd 14 77 | cat -e
+7$
+student@ubuntu:~/student/gcd$ ./gcd 17 3 | cat -e 
+1$
+student@ubuntu:~/student/gcd$ ./gcd | cat -e
+$
+student@ubuntu:~/student/gcd$
+```