Port threading design doc to in-tree docs, start a README for design docs in the tree.

docs/README.md

 # Chromium docs
 
 This directory contains chromium project documentation in
 [Gitiles-flavored Markdown](https://gerrit.googlesource.com/gitiles/+/master/Documentation/markdown.md).
 It is automatically
 [rendered by Gitiles](https://chromium.googlesource.com/chromium/src/+/master/docs/).
 
 If you add new documents, please also add a link to them in the Document Index
 below.
 
@@ skipping 39 matching lines @@
 *   [Common Build Tasks](common_build_tasks.md) - Recipes for slightly more
     advanced build tasks
 *   [Chrome Component Build](component_build.md) - Faster builds using more
     libraries
 *   [Using the BuildRunner](using_build_runner.md) - Scripts that extract build
     stops from builders and runs them locally on a slave
 *   [Cr User Manual](cr_user_manual.md) - Manual for `cr`, a tool that tries to
     hide some of the tools used for working on Chromium behind an abstraction
     layer
 
+### Design Docs
+*   See [design/README.md](design/README.md)
+
 ### Integrated Development Environment (IDE) Set Up Guides
 *   [Android Studio](android_studio.md) - Android Studio for Android builds
 *   [Eclipse for Android](eclipse.md) - Eclipse for Android
 *   [Eclipse for Linux](linux_eclipse_dev.md) - Eclipse for other platforms
     (This guide was written for Linux, but is probably usable on Windows/MacOS
     as well)
 *   [Qt Creator](qtcreator.md) - Using Qt Creator as an IDE or GUI debugger
 *   [Setting up Visual Studio Code](vscode.md) - Visual Studio Code
 *   [EMACS Notes](emacs.md) - EMACS commands/styles/tool integrations
 *   [Atom](atom.md) - Atom multi-platform code editor
@@ skipping 231 matching lines @@
 *   [Linux `SUID` Sandbox](linux_suid_sandbox.md) - Sandboxing renderers using a
     SUID binary on Linux
 *   [Linux `SUID` Sandbox Development](linux_suid_sandbox_development.md) -
     Development on the above system.
 *   [Linux PID Namespace Support](linux_pid_namespace_support.md)
 *   [Vanilla msysgit workflow](vanilla_msysgit_workflow.md) - A workflow for
     using mostly vanilla git on Windows.
 *   [Old Chromoting Build Instructions](old_chromoting_build_instructions.md)
 *   [Old Options](chrome_settings.md) - Pre-Material Design chrome://settings
     notes.