Generate the proto JSON converter for DOM distiller

third_party/dom_distiller_js/protoc_plugins/README.md

Index: third_party/dom_distiller_js/protoc_plugins/README.md
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+# protoc plugin for JSON converter
+
+These protoc plugins use a simple JSON encoding.
+
+An instance of the following protobuf:
+
+```
+message Foo {
+  message Bar {
+    repeated string rabbits = 1;
+  }
+  optional string cat = 1;
+  repeated int32 dog = 2;
+  optional Bar rabbit_den = 3;
+}
+```
+
+could be encoded to something like:
+
+```
+{
+  "1": "kitty",
+  "2": [4, 16, 9],
+  "3": { "1": ["thumper", "oreo", "daisy"] }
+}
+```
+
+Only a limited part of the protocol buffer IDL is supported.
+
+*   Supported field types: `float`, `double`, `int32`, `bool`, `string`, `message`, and `enum`
+
+*   Supported field rules: `optional`, `repeated`
+
+*   Unsupported features:
+
+    *   default values
+    *   imports
+    *   extensions
+    *   services
+    *   non-file-level options