Add project overview

Author: Havret

README.md

@@ -5,6 +5,10 @@
 
 This library provides support for [Google.Protobuf](https://www.nuget.org/packages/Google.Protobuf) types to [System.Text.Json](https://www.nuget.org/packages/System.Text.Json/).
 
+By default classes generated from `*.proto` files don't work particularly well with `System.Text.Json`. You can properly serialize and deserialize only very simple contracts (only scalar value types work properly). When your schema contains some more advanced structures like repeated fields, maps, oneofs, or optional fields the results are far from acceptable. 
+
+You may try to write a custom JsonConverter that will utilize `Google.Protobuf.JsonFormatter` and `Google.Protobuf.JsonParser` to do all the heavy lifting. Unfortunately, this solution falls short in terms of performance. This library provides high-performance and low-allocating types that serialize `Google.Protobuf` objects to JavaScript Object Notation (JSON) text and deserialize JSON text to `Google.Protobuf` objects.
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 ## Performance
 
 ``` ini