Add warning about Lucene/Elasticsearch upgrades
Add warning about Lucene/Elasticsearch upgrades

When upgrading Lucene or Elasticsearch, we need to make sure
they are compatible with each other. Elasticsearch is built
for a specific version of Lucene and will not run if Lucene
is the wrong version.

Change-Id: I4b6cb82dd0540b4c2d44718e445c316e5e9c274d

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