Include many-to-one example

[adamrtalbot]

It is useful to run a single pipeline after a number of pipelines have completed, i.e. a many-to-one relationship. This is possible with node-RED but we should provide an example for users to copy.

[ewels]

I think that the Join node should be the way to do this.

May need to be a bit clever about knitting together IDs somehow, to be able to handle multiple concurrent sets of pipeline executions.