Explain Lumberjack design decision: shipping the log is more important than avoiding duplication

Hey Jordan, your response to "Duplicate events in ES index ... and some rant" on the mailing list was very revealing to me. I've been using Lumberjack for a while and didn't realize that. Yeah I rarely ever read C code :-)

I think it would make sense to explain this behaviour in the readme file.

What do you think? Please let me know if you want me to improve anything in this PR.