Add content/OWNER guidelines

Change-Id: If360867e6c78f4d3b700d944181063e6c618a5a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2242833
Auto-Submit: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779000}

content/OWNERS

+# src/content is a large and complex component. In general, files should be in
+# subdirectories that have specific owners who are the experts in reviewing that
+# code. Top level content/OWNERS are reviewers who are qualified to review
+# changes across all of content and are responsible for its architecture. They
+# will defer to subdirectory owners as needed. As such we expect that a
+# content/OWNER has made substantial contributions to content/ in the recent
+# past, specifically both in the browser and renderer subdirectories as they
+# encapsulate the main architecture, and demonstrated overall knowledge of the
+# content architecture by doing code reviews for changes that affect different
+# parts of content even before being a content/OWNER.
+
 alexmos@chromium.org
 avi@chromium.org
 clamy@chromium.org