doc: add asynchronous execution example

From #5806, this is the same async example added to the v0.10 branch with the isolates code removed.

When the dust settles from #5804 I'll fix up the examples and submit a new PR with updates to master. I'm trying to maintain 0.11 compat in the [0.11 branch of node-addon-examples](https://github.com/rvagg/node-addon-examples/tree/v0.11/).