goprotobuf: Mention Go 1.1 requirement in README.

Fixes #48.

R=r, dave
CC=golang-codereviews
https://codereview.appspot.com/45600043

README

@@ -2,6 +2,8 @@ Go support for Protocol Buffers - Google's data interchange format
 Copyright 2010 The Go Authors.
 http://code.google.com/p/goprotobuf/
 
+This package and the code it generates requires at least Go 1.1.
+
 This software implements Go bindings for protocol buffers.  For
 information about protocol buffers themselves, see
 	http://code.google.com/apis/protocolbuffers/