Xcode: move Xcode3 specific files into sub directory

- makes space for Xcode4 files
- added README

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5720059

misc/xcode/3/README

+This directory contains files for Go syntax highlighting in Xcode 3.x.
+See the comments go.pbfilespec and go.xclangspec for installation
+instructions.