Add README

Author: swallez

rest5-client/README.md

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+# rest5-client
+
+The Elasticsearch Java client historically used a "Low Level Rest Client" (LLRC) based on Apache http client version 4 to handle http communications and distribution of requests among the nodes of a cluster.
+
+This directory is a port of this client to Apache http client version 5 that is mostly a drop-in replacement, except for the initialization phase.
+
+It is provided as an independent library as a convenience to users who have an existing code based on LLRC and would like to migrate to the more modern Apache http 5 library.