[Docs] comment on udp and effected pods in readme

[paulgmiller]

Wasn't clear that udp didn't get send every packet to user space so after I confirmed that established also ignored tcp flows I thought I'd update the readme.

Also the readme made it seem like ignoring non selected pods was a future optimization but it seems already one so I updated that paragraph too.

Somewhat related to #99

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[aojea]

/ok-to-test
/lgtm
/approve

Thanks

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