README: expand the description

Provide a bit more context in the README of what this is and how to find
more information. This is necessary because many people use GitHub as a
discovery mechanism.

README.md

@@ -2,7 +2,7 @@
 
 [![Build Status](https://travis-ci.org/grpc/grpc-go.svg)](https://travis-ci.org/grpc/grpc-go) [![GoDoc](https://godoc.org/google.golang.org/grpc?status.svg)](https://godoc.org/google.golang.org/grpc)
 
-The Go implementation of [gRPC](https://github.com/grpc/grpc)
+The Go implementation of [gRPC](http://www.grpc.io/): A high performance, open source, general RPC framework that puts mobile and HTTP/2 first. For more information see the [gRPC Quick Start](http://www.grpc.io/docs/) guide.
 
 Installation
 ------------