document how to do a start/defer style profile dump

I want to do multiple runs of my code, dump each to a file and then open them in a flamgraph viewer, normally I do this:
```
f, _ := os.Create("cpu.prof")
pprof.StartCPUProfile(f)
defer pprof.StopCPUProfile()
```

... here it seems to be:
```
f, _ := os.Create("cpu.prof")
stop := fgprof.Start(f, "pprof")
defer stop()
```

but some proper Readme docs would be great