Add example

Based on an example provided by Ryan in [1]

[1]: ryankurte#1 (comment)

examples/osrng.rs

@@ -0,0 +1,19 @@
+use rand::{RngCore, rngs::OsRng};
+use rngcheck::{nist::*, helpers::*};
+
+fn main() {
+    let mut rng = OsRng;
+
+    // Fetch random bytes for tests
+    let mut a = [0xFF; 100];
+    rng.fill_bytes(&mut a);
+
+    // Check we filled -something- before attempting more in-depth tests
+    if &a[..2] == &[0xFF; 2] && &a[a.len() - 2..] == &[0xFF; 2] {
+        panic!("RNG seems to have no-op'ed");
+    }
+
+    // Run NIST frequency checks
+    println!("Monobit result: {:?}", nist_freq_monobit(BitIter::new(&a)));
+    println!("Freq block result: {:?}", nist_freq_block(BitIter::new(&a), 10));
+}