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

deploytool/api/deploy/checkout.pb.go

 // Code generated by protoc-gen-go.
 // source: github.com/luci/luci-go/deploytool/api/deploy/checkout.proto
 // DO NOT EDIT!
 
 /*
 Package deploy is a generated protocol buffer package.
 
 It is generated from these files:
         github.com/luci/luci-go/deploytool/api/deploy/checkout.proto
         github.com/luci/luci-go/deploytool/api/deploy/component.proto
@@ skipping 32 matching lines @@
 // This is a compile-time assertion to ensure that this generated file
 // is compatible with the proto package it is being compiled against.
 // A compilation error at this line likely means your copy of the
 // proto package needs to be updated.
 const _ = proto.ProtoPackageIsVersion2 // please upgrade the proto package
 
 // *
 // Source layout configuration file.
 //
 // Each Source checkout may include a textproto layout file named
-// "luci-deploytool.cfg". If present, this file will be loaded and used to
+// "luci-deploy.cfg". If present, this file will be loaded and used to
 // integrate the source into the deployment.
 //
 // Uncooperative repositories, or those not owned by the author, may have the
 // same effect by specifying the SourceLayout in the Source's layout definition.
 type SourceLayout struct {
         // * The source initialization operations to execute, in order.
         Init []*SourceLayout_Init `protobuf:"bytes,1,rep,name=init" json:"init,omitempty"`
         // * Go Paths to add to this repository.
         GoPath []*GoPath `protobuf:"bytes,10,rep,name=go_path,json=goPath" json:"go_path,omitempty"`
 }
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 }