add goroutines example

There are often questions about how to use this lib with Goroutines (i.e. #3 , DataDog#49). There's an [extensive blogpost](https://www.datadoghq.com/blog/engineering/cgo-and-python/#the-dreadful-global-interpreter-lock) on the topic, but it's for `sbinet/go-python`. We should add an example (in the `examples/` dir).  