Add small README for each project being released.

Signed-off-by: Yury-Fridlyand <yuryf@bitquilltech.com>

src/OpenSearch.Client.JsonNetSerializer/README.md

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+An auxiliary tool used by `OpenSearch.Client` to serialize and deserialize data.

src/OpenSearch.Client/README.md

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+`OpenSearch.Client` is a high level OpenSearch .NET client that still maps very closely to the original OpenSearch API. All requests and responses are exposed through types, making it ideal for getting up and running quickly.
+
+Under the covers, `OpenSearch.Client` uses the `OpenSearch.Net` low level client to dispatch requests and responses, using and extending many of the types within `OpenSearch.Net`. The low level client itself is still exposed on the high level client through the `.LowLevel` property.

src/OpenSearch.Net.VirtualizedCluster/README.md

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+A utility used by unit tests of `OpenSearch.Net` and `OpenSearch.Client`. It allows to create a fake virtual OpenSearch cluster to test and debug REST API calls.

src/OpenSearch.Net/README.md

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+A low-level, dependency free OpenSearch .Net client.
+
+It provides a one-to-one mapping with the OpenSearch REST API. The client is completely matches the official REST API specification and supports all OpenSearch features. The client also has support for load balancing and cluster failover and all client method calls have both synchronous and asynchronous variants