Document user data directory logic.

Incorporates all of https://www.chromium.org/user-experience/user-data-directory
which can now be a redirect.  Adds some things that weren't documented,
including guidance on the new $CHROME_CONFIG_HOME variable.

Bug: 726016
Change-Id: I91944f04ffbcfbd268c8d18d2121ffd1b5f54a70
Reviewed-on: https://chromium-review.googlesource.com/558653
Reviewed-by: Lei Zhang <[email protected]>
Commit-Queue: Steve Kobes <[email protected]>
Cr-Commit-Position: refs/heads/master@{#484358}
diff --git a/docs/README.md b/docs/README.md
index b4601df..d9f77bd 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -294,6 +294,8 @@
     used to automatically set proxy settings.
 *   [Installing Chromium OS on VMWare](installation_at_vmware.md) - How to
     install Chromium OS on VMWare.
+*   [User Data Directory](user_data_dir.md) - How the user data and cache
+    directories are determined on all platforms.
 
 ### Probably Obsolete
 *   [Old ChromeOS build instructions](old_chromeos_build_instructions.md)