Add "Threading and Tasks in Chrome" documentation.

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 79 matching lines @@
 *   [Using clang-format on Chromium C++ Code](clang_format.md) - Various ways to
     invoke clang-format on C++ code
 *   [Clang Tidy](clang_tidy.md) - Support for the `clang-tidy` tool in Chromium
 *   [Updating Clang Format Binaries](updating_clang_format_binaries.md) - How up
     update the clang-format binaries that come with a checkout of Chromium
 
 ### General Development
 *   [Code Reviews](code_reviews.md) - Code review requirements and guidelines
 *   [Closure Compilation](closure_compilation.md) - The _Closure_ JavaScript
     compiler
+*   [Threading and Tasks in Chrome](threading_and_tasks.md) - How to run tasks
+    and handle thread safety in Chrome.
 *   [Callback<> and Bind()](callback.md) - All about Callbacks, Closures, and
     Bind().
 *   [Views Platform Styling](ui/views/platform_style.md) - How views are styled
     to fit in different native platforms
 *   [Tab Helpers](tab_helpers.md) - Using WebContents/WebContentsObserver to add
     features to browser tabs.
 *   [Adding third_party Libraries](adding_to_third_party.md) - How to get code
     into third_party/
 *   [Graphical Debugging Aid for Chromium Views](graphical_debugging_aid_chromium_views.md) -
     Visualizing view trees during debugging
@@ skipping 194 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.