Add `S3EventNotifier` example
### Motivation:
There is currently no example regarding an AWS event triggered lambda,
such as a lambda that gets invoked on an S3 object upload. I've had to
look for a while to figure out that the `AWSLambdaEvents` exists and
that it can be used for that (had to find out via a Java example! 😜) so
I think this could be useful and a somewhat common use case
### Modifications:
Added an example of a lambda that gets triggered when an object gets
uploaded to an S3 bucket.
I have not described how to set up the actual S3 event in AWS because I
figured if you needed such a lambda you'd know, but I can describe that
too in the README if needed
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Co-authored-by: Sébastien Stormacq <sebastien.stormacq@gmail.com>
Sébastien Stormacq
Paul ToffoloniandSébastien Stormacq