Add explicit instructions to run elastic search to readme.

[jackiekazil]

I would have submitted this as a pull request, but I am not sure if you had a preferred way of doing this or an opinion on where this should go in the Readme.

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When importing data, I got a connection error and realized it was because I didn't have Elastic search running. Maybe add something to the Setup section?

Details if you decide to add something specific to the readme.....

If you get this error while importing data:

{"error":"Put http://localhost:9200/hive/assignments/_mapping: dial tcp 127.0.0.1:9200: connection refused"}

Make sure that elastic search is running. From the Homebrew Elastic Search install:

To have launchd start elasticsearch at login:
    ln -sfv /usr/local/opt/elasticsearch/*.plist ~/Library/LaunchAgents
Then to load elasticsearch now:
    launchctl load ~/Library/LaunchAgents/homebrew.mxcl.elasticsearch.plist
Or, if you don't want/need launchctl, you can just run:
    elasticsearch --config=/usr/local/opt/elasticsearch/config/elasticsearch.yml