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

…docs in the tree.

This is a straight up port of https://www.chromium.org/developers/design-documents/threading, with minimal editing only for style consistency and some grammar errors.

Review-Url: https://codereview.chromium.org/2822353002
Cr-Commit-Position: refs/heads/master@{#466049}

docs/README.md

@@ -57,6 +57,9 @@ git cl patch <CL number or URL>
     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

docs/design/README.md

@@ -0,0 +1,13 @@
+# Chromium Design 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/).
+
+Documents here have been imported
+from [the Project site](https://www.chromium.org/developers/design-documents).
+As of this writing, the vast majority of docs have not been imported yet.
+
+* [Threading](threading.md) - Preferred ways to use threading, and library
+  support for concurrency.