Add notes how to install depended autoconf tool.
Add notes how to install depended autoconf tool.

mingchen/protobuf-ios,shenhzou654321/protobuf-ios,mingchen/protobuf-ios,shenhzou654321/protobuf-ios,shenhzou654321/protobuf-ios,mingchen/protobuf-ios,shenhzou654321/protobuf-ios,mingchen/protobuf-ios,shenhzou654321/protobuf-ios,shenhzou654321/protobuf-ios,mingchen/protobuf-ios,mingchen/protobuf-ios