Add README files to content and content/public.

NOTRY=true

Review URL: https://codereview.chromium.org/388053003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283556 0039d316-1c4b-4281-b951-d872f2087c98

content/README

+Content is the core code needed to render a page using a multi-process sandboxed
+browser.
+
+Developer documentation:
+http://www.chromium.org/developers/content-module
+

content/public/README

+Content API design:
+http://www.chromium.org/developers/content-module/content-api
+