Adding usage example for BENCHMARK_DRAW_LINE()

Summary:
Yes, I admit, I was dumb enough to try it in a benchmark's
body.

Test Plan: review

Reviewed By: tudorb@fb.com

FB internal diff: D622613

folly/docs/Benchmark.md

@@ -178,6 +178,20 @@ group or for whatever reason), use `BENCHMARK_DRAW_LINE()`. The line
 fulfills a purely aesthetic role; it doesn't interact with
 measurements in any way.
 
+``` Cpp
+    BENCHMARK(foo) {
+      Foo foo;
+      foo.doSomething();
+    }
+
+    BENCHMARK_DRAW_LINE();
+
+    BENCHMARK(bar) {
+      Bar bar;
+      bar.doSomething();
+    }
+```
+
 ### Suspending a benchmark
 ***