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

docs/design/README.md

Index: docs/design/README.md
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+# 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.