Fix README example to not run workflow on delete

[sambarnes]

Great tool here! Integrated it this morning into my own project and was surprised how easy and effective it is.

One thing that I noticed in my trials: the example in the README will run the workflow for both creation & deletion of a comment containing a trigger phrase. This seems like it would be (more often than not) undesirable for most use cases.

Looking into it more, I saw that the issue_comment workflow event has created, edited, and deleted types. Proposing this small change to the demo to help the next person become immediately aware that they can explicitly limit the runs to creation only.

Thanks again for such a great tool!