Worker: Add README file to core/workers

Bug: 837127
Change-Id: Iee15da96b578ca1385440716ae5b733f30559f68
Reviewed-on: https://chromium-review.googlesource.com/1029564
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553937}

third_party/blink/renderer/core/workers/README.md

+This directory contains the base implementation of all worker and worklet types. Also, this contains the implementation of the [Web Workers API](https://html.spec.whatwg.org/multipage/workers.html#workers) (Dedicated Worker and Shared Worker) and [Worklets API](https://drafts.css-houdini.org/worklets/).
+
+# Worker / Worklet types
+
+- Workers are divided into 2 types:
+  - **In-process workers (Dedicated Workers)**: A worker of this type always runs in the same renderer process with a parent document that starts the worker.
+  - **Out-of-process workers (Shared Workers and Service Workers)**: A worker of this type may run in a different renderer process with a parent document that starts the worker.
+- Worklets are divided into 2 types:
+  - **Main thread worklets (Paint Worklets and Layout Worklets)**: A worklet of this type runs on the main thread.
+  - **Threaded worklets (Audio Worklets and Animation Worklets)**: A worklet of this type runs on a worker thread.
+
+Worklets always run in the same renderer process with a parent document that starts the worklet like the in-process-workers.
+
+# Naming conventions
+
+Classes in this directory are named with the following conventions, there're still some exceptions though.
+
+- `WorkerOrWorklet` prefix: Classes commonly used for workers and worklets (e.g., `WorkerOrWorkletGlobalScope`).
+- `Worker` / `Worklet` prefix: Classes used for workers or worklets (e.g., `WorkerGlobalScope`).
+- `Threaded` prefix: Classes used for workers and threaded worklets (e.g., `ThreadedMessagingProxyBase`).
+- `MainThreadWorklet` prefix: Classes used for main thread worklets (e.g., `MainThreadWorkletGlobalScope`).
+
+Thread hopping between the main (parent) thread and a worker thread is handled by proxy classes.
+
+- `MessagingProxy` is the main (parent) thread side proxy that communicates to the worker thread.
+- `ObjectProxy` is the worker thread side proxy that communicates to the main (parent) thread. `Object` indicates a worker/worklet JavaScript object on the parent execution context.
+
+# References
+
+- [Worker / Worklet Internals](https://docs.google.com/presentation/d/1GZJ3VnLIO_Pw0jr9nRw6_-trg68ol-AkliMxJ6jo6Bo/edit?usp=sharing) (April 19, 2018)