Add tool to generate PropertyConverter implementations for proto messages.

tools/proto-gae/README.md

Index: tools/proto-gae/README.md
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+proto-gae
+=========
+
+proto-gae is a simple `go generate`-compatible tool for generating
+"github.com/luci/gae/service/datastore".PropertyConverter implementation for
+`protoc`-generated message types. This allows you to embed `proto.Message`
+implementations into your datastore models.
+
+The generated implementations serialize to/from the binary protobuf format into
+an unindexed []byte property.
+
+
+Example
+-------
+
+#### path/to/mything/protos/mything.proto
+```protobuf
+syntax = "proto3";
+package protos;
+message MyProtoThing {
+  my string = 1;
+  proto int64 = 1;
+  thing float = 1;
+}
+```
+
+#### path/to/mything/protos/gen.go
+```go
+package protos
+// assume github.com/luci/luci-go/tools/cmd/cproto is in $PATH. Try it, it's
+// awesome :).
+
+//go:generate cproto
+//go:generate proto-gae -type MyProtoThing
+```
+
+#### path/to/mything/thing.go
+```go
+package mything
+
+import "path/to/package/protos"
+
+type DatastoreModel struct {
+  // This will now 'just work'; ProtoMessage will round-trip to datastore as
+  // []byte.
+  ProtoMessage protos.MyProtoThing
+}
+```