README.md: initial commit

Change-Id: Ifa901a402a3d2c9e4c35d447b77224861cb2ffd4
Reviewed-on: https://go-review.googlesource.com/127335
Reviewed-by: Damien Neil <dneil@google.com>

README.md

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+# Next Generation Go Protocol Buffers
+
+**WARNING**: This repository is in active development. There are no guarantees
+about API stability. Breaking changes *will* occur until a stable release is
+made and announced.
+
+This repository is for the development of the next major Go implementation
+of protocol buffers. This library makes breaking API changes relative to the
+[existing Go protobuf library](https://github.com/golang/protobuf/tree/master).
+Of particular note, this API aims to make protobuf reflection a first-class
+feature of the API and implements the protobuf ecosystem in terms of reflection.
+
+# Design Documents
+
+List of relevant design documents:
+
+* [Go Protocol Buffer API Improvements](https://goo.gl/wwiPVx)
+* [Go Reflection API for Protocol Buffers](https://goo.gl/96gGnE)
+
+# Reporting Issues
+
+Issues regarding the new API can be filed at
+[github.com/golang/protobuf](https://github.com/golang/protobuf/issues).
+Please use a `APIv2:` prefix in the title to make it clear that
+the issue is regarding the new API work.