[C/en] Added tidbit about fall-though in switch statements.

Another pitfall, as not all languages have fall-through in switches.

c.html.markdown

@@ -318,6 +318,12 @@ int main (int argc, char** argv)
   case 1:
     printf("Huh, 'a' equals 1!\n");
     break;
+    // Be careful - without a "break", execution continues until the
+    // next "break" is reached.
+  case 3:
+  case 4:
+    printf("Look at that.. 'a' is either 3, or 4\n");
+    break;
   default:
     // if `some_integral_expression` didn't match any of the labels
     fputs("error!\n", stderr);