elasticsearch-model/README - update index_document with __elasticsearch_...

[notapatch]

I was trying to add re-indexing hooks to my program, by following the elasticsearch-model/README, and was getting the error (undefined local variable or method `index_document'). I changed the example to this (see below) and things worked as expected:

    after_commit on: [:create, :update] do
      __elasticsearch__.index_document
    end

    after_commit on: [:destroy] do
      __elasticsearch__.delete_document
    end

I wondered if the README needed updating? If so, I have added this pull request.

[karmi]

@RichardWigley Yeah!, that's true -- can you please sign the CLA so I can merge the contribution?

[notapatch]

I googled elasticsearch and CLA and signed a CLA - I hope that is what was required.

[karmi]

@RichardWigley You've found the correct thing :) -- commit merged, thanks!