deploytool: Add README.md, migrate docs to it.

deploytool/cmd/luci_deploy/doc.go

Index: deploytool/cmd/luci_deploy/doc.go
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+// Copyright 2016 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+// Package main contains the entry point code for the LUCI Deployment Tool
+// ("luci_deploy"). This tool is a command-line interface designed to perform
+// controlled and automated deployment of LUCI (and LUCI-compatible) services.
+//
+// "luci_deploy" is Linux-oriented, but may also work on other platforms. It
+// leverages external tooling for many remote operations; it is the
+// responsibility of the user to have suitable versions of that tooling
+// installed and available on PATH.
+package main