Remove or document the restriction that index must have at most 1 shard + 1 replica for the plugin to work

[cristi-maco]

There is a hard validation in code (in FairRescoreBuilder.java) that checks that the index has only 1 shard (+1 replica).
This means that the plugin is unusable for bigger indexes, which for performance (and/or reliability) reasons need more than 1 shard (or 1 replica) when deployed (like in a production environment).

If this is an unavoidable limitation of the algorithm or the elasticsearch api used by it, please document it clearly in the readme / userguide (currently there is no mention about this), as for many potential users I think this is a very important consideration, which should be known upfront.

If on the other hand this is an avoidable limitation, and the code can be changed/improved so it works with multiple shards, then please consider improving it in this direction.

[chatox]

Sorry for the late reply. I think the restriction is not needed, but we will look into it.