This commit adds an example to the runtime/trace package
on how to use the trace.Start and trace.Stop functions
to trace the execution of a Go program and write
its trace output to a file.

Change-Id: Idf920398f1c3b9d185af9df5ce9293f2361db022
Reviewed-on: https://go-review.googlesource.com/51170Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>