Add a spouts directory and README for spout implementations

[nwangtw]

Heron community has been interested in maintaining spout implementations for users. There have been discussions in slack channel (#general) and dev@ mailing list.

There are multiple options and each has its own pros and cons. This PR is just a start of the effort to unblock contributors. The directory might be migrated to a different place after a final decision is made.

Please feel free to comment or join the discussion in dev@heron.apache.org.

[huijunw]

there was a contrib dir, which included spouts.
https://github.com/apache/incubator-heron/tree/0.15.0/contrib
i think we may recover the contrib dir, and incubate codes such as spouts from there till they are migrated to other places. thoughts?

[nwangtw]

there was a contrib dir, which included spouts.
https://github.com/apache/incubator-heron/tree/0.15.0/contrib
i think we may recover the contrib dir, and incubate codes such as spouts from there till they are migrated to other places. thoughts?

Cool. Thanks!

[nwangtw]

I am thinking maybe we should have both spouts and bolts. Both are for reusable components anyway. For example KafkaSpout reads from Kafka and KafkaBolt could be the writer.

[simingweng]

@nwangtw definitely make sense to have both Spout and Bolt, Apache Storm has the same thing.

[nwangtw]

@simingweng thx for the input!