Contribute a small query device example?

I think a tiny example such as 

```
$ cat devices.c 
#include <omp.h>
#include <stdio.h>
int main() {
  printf("There are %d devices\n", omp_get_num_devices());
}
```

Would be a useful add to the tutorial. I'd do a PR about it, but I don't have access to all the compilers (only gcc and PGI, and only on linux) so I cannot test if it works as expected on the other compilers/platforms