Include original README section for project details

Author: stepancheg

README.md

@@ -20,6 +20,8 @@
 This implementation is approaching end of life. Official protobuf implementation (not based on this implementation) is on the way,
 that will be released as protobuf=4 soon. Follow progress in [protobuf repository](https://github.com/protocolbuffers/protobuf/tree/main/rust).
 
+# Original readme
+
 [Protobuf](https://developers.google.com/protocol-buffers/docs/overview) implementation in [Rust](https://www.rust-lang.org/).
 
 * Written in pure rust