Merge pull request #82 from fsharpasharp/GCD

I have merged your code. Can you put in some comments.

misc/GCD.c

@@ -0,0 +1,15 @@
+#include <stdio.h>
+
+// Euclid's algorithm
+int GCD(int x, int y) {
+    if (y == 0)
+        return x;
+    return GCD(y, x%y);
+}
+
+int main() {
+    int a, b;
+    printf("Input two numbers:\n");
+    scanf("%d %d", &a, &b);
+    printf("Greatest common divisor: %d\n", GCD(a, b));
+}