Added a README for the Chrome Perfetto integration

R=primiano@chromium.org

No-Try: true
Change-Id: Ia6642d8576939ba28492ddada335bb10e77e802f
Reviewed-on: https://chromium-review.googlesource.com/1031516
Commit-Queue: oysteine <oysteine@chromium.org>
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554872}

services/tracing/perfetto/README.md

+# Perfetto in Chrome
+
+[TOC]
+
+## Overview
+
+Perfetto is a project intended to provide a common platform for performance
+instrumentation and logging across Chrome and Android, and this directory
+contains the  code integrating the Perfetto library into Chrome. Specifically
+it provides a Mojo-based transportation layer which any individual new data source can
+build on to send logging protos to Perfetto, and an implementation which lets
+`about://tracing` generate a Chrome Tracing trace  (`TRACE_EVENT0`, etc) using
+Perfetto rather than `/base/trace_event/trace_log.cc` as a backend.
+
+The library itself lives in [AOSP](https://android.googlesource.com/platform/external/perfetto/)
+and is rolled in [/third_party/chrome/](https://cs.chromium.org/chromium/src/third_party/perfetto/).
+
+## Perfetto Documentation
+
+[Project page](https://android.googlesource.com/platform/external/perfetto/+/master/README.md)
+
+[Life of a Perfetto tracing Session](https://android.googlesource.com/platform/external/perfetto/+/master/docs/life-of-a-tracing-session.md)
+
+[Internal documentation](http://go/perfetto-project)
+
+## Directory Structure
+
+```
+//services/tracing/                   <-- Perfetto is embedded by the tracing service
+              /perfetto/              <-- Internal service implementation code
+              /public/
+                     /cpp/perfetto    <-- C++ client libraries used by the data source providers.
+                     /mojom/          <-- Mojom interfaces
+//third_party/perfetto/               <-- DEPS-rolled external library
+```
+
+## Adding a new data source
+
+A data source is a provider of a specific type of data in the form of protobufs,
+like Chrome Trace Events, memory-infra memory dumps, netlog, etc. It registers itself
+with Perfetto with a given string identifier (e.g. `org.chromium.trace-event`) and if enabled
+by the central Perfetto service, writes its protos into the provided Perfetto TraceWriter(s).
+
+These data source providers can live in any child process, and the Mojo transportation layer
+will take care of the details of passing them to the central service through shared memory
+buffers.
+
+To add a new data source:
+
+* Add a new string identifier in [perfetto_service.mojom](/services/tracing/public/mojom/perfetto_service.mojom).
+* Register the data source in [ProducerHost::OnConnect](/services/tracing/perfetto/producer_host.cc).
+* Set up the data source in [ProducerClient::CreateDataSourceInstance](/services/tracing/public/cpp/perfetto/producer_client.cc).
+* Tear down the data source in [ProducerClient::TearDownDataSourceInstance](/services/tracing/public/cpp/perfetto/producer_client.cc).
+* For each thread that wants to log a proto, use a separate TraceWriter created using
+  [ProducerClient::CreateTraceWriter](/services/tracing/public/cpp/perfetto/producer_client.cc).
+
+## Contact
+
+For any questions about Perfetto in Chrome or adding a new data source, please
+start a thread on [tracing@chromium.org](https://groups.google.com/a/chromium.org/forum/#!forum/tracing).