Add mdjones to touchless OWNERS.

Also added some general guidance on which reviewers to add for which
changes.

Change-Id: Idb8f908ff044e82e40e13dc561f0263c2965bb7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1634558Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664374}

chrome/android/touchless/OWNERS

+# General
+mdjones@chromium.org
+mthiesse@chromium.org
+yfriedman@chromium.org
+
+# NTP/Explore
 chili@chromium.org
 dewittj@chromium.org
-mthiesse@chromium.org
 pavely@chromium.org
 skym@chromium.org
 wylieb@chromium.org
-yfriedman@chromium.org
 
 # COMPONENT: Mobile>Touchless
 # OS: Android