Document how to deal with "ERROR: Hotspot compilation occurred during timing. Warmup is likely insufficent." when running tests

```
What steps will reproduce the problem?
1. I run the code below with no additional arguments
2.
3.

What is the expected output? What do you see instead?
Caliper completes without printing to stderr.
Instead, during one of the experiments, Calper prints:

"ERROR: Hotspot compilation occurred during timing. Warmup is likely
insufficent."

What version of the product are you using? On what operating system?
a3703ae4544b3952e0944fd8d7e3dcd16f72c3fc

java version "1.7.0_17"
Java(TM) SE Runtime Environment (build 1.7.0_17-b02)
Java HotSpot(TM) 64-Bit Server VM (build 23.7-b01, mixed mode)

Please provide any additional information below.

code:

import com.google.caliper.Benchmark;
import com.google.caliper.runner.CaliperMain;

public class TestFinally extends Benchmark {
public long timeNanoTime(int reps) {
long ret = 0;
for (int i = 0; i < reps; i++) {
ret += System.nanoTime();
}
return ret;
}

public long timeCurrentTimeMillis(int reps) {
long ret = 0;
for (int i = 0; i < reps; i++) {
ret += System.currentTimeMillis();
}
return ret;
}

public static void main(String[] args) throws Exception {
CaliperMain.main(TestFinally.class, args);
}
}

```

Original issue reported on code.google.com by `agrothberg` on 8 Apr 2013 at 12:03